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Brain Size

Intelligence is very roughly correlatable with brain size. As humans evolved, brain size increased dramatically. We can see at a glance in the image at left the increase in brain size and shape over 3.5 million years. The A. afarensis skull at left is approximately 400 cc, about the same size as a chimp's brain. The million-year old Homo erectus skull in the center is 1200 cc, and the modern human skull at right holds a brain of 1400 cc. The shape of the homind skull changes dramatically as well, to accommodate the development of a forebrain. Note that A. afarensis on the left, one of <![if !vml]><![endif]>the earliest hominid species, has almost no forehead at all.

The image shows, from left to right, a monkey, chimp, and human brain. As the human brain developed, the number and depth of its creases and convolutions also increased. <![if !vml]><![endif]>

The fossil evidence allows us to trace the gradual increase in brain size over the past two or two and a half million years with some degree of precision. The average brain size of Homo habilis, far left above, who lived approximately 2 million years ago, was 750 cc. Homo erectus (next skull to the right) shows this transition most dramatically, indicating that most of the evolutionary increase in brain size took place during the life of this species. Early Homo erectus in Africa (from about 1.7 to 1 million years ago) averaged 900 cc in brain size, but later Homo erectus specimens from .5 million years ago average 1100-1200 cc, which falls within the range of the brain size of modern humans. The earliest or archaic forms of Homo sapiens, the species to which we belong (top center skull), dates to 300,000-400,000 years ago and averages over 1200 cc. The Neanderthal skull, second from right, has a brain size of 1500 cc, which is actually larger than the brains of most modern humans. The average for ourselves, Homo sapiens sapiens, is around 1400 cc. <![if !vml]><![endif]>

While there is rough correlation between brain size in relation to overall body size and intelligence, scientists caution that that correlation is very loose. Human brain size varies considerably, just as body size does. The brain size of recognized "geniuses" can vary from 1000 cc to 2000 cc in modern humans. Clearly, one has to examine far subtler features of the brain to understand the relations between physical characteristics and intellectual capacities or between brain physiology and social or cultural behavior.

Surprisingly, a big brain is not an obvious evolutionary advantage--at least not immediately. For example, a large brain requires an inordinate amount of care and feeding--a diet high in protein--and exquisite temperature control in order to function properly. Increases in brain size would therefore drive changes in early human diets, because of the need to increase the intake of protein. While human species remained ominivorous, a preference for meat-eating did in fact occur over time. About 25% of our metabolism is devoted to brain function, which represents a huge investment of energy--and therefore a huge risk in terms of the overall chances for survival of the species. Overheating the large human brain in the hot climate of east Africa was potentially fatal; to see how our ancestors evolved solutions to this problem see the section on heat diffusion.

The Human Face and Eyes

The human face seems so uniquely "ours" that we may be surprised at its similarities with other forms of life. Our faces are adapted to the intense and complicated social lives that we live as humans; accordingly, they are "signboards" capable of sending highly complex and nuanced messages. So, in many ways, is the chimp face, which has the same number of facial muscles and forms many of the <![if !vml]><![endif]>same basic expressions as we do (frowns, grins, threatening grimaces, etc). The male, above left, even has facial hair (chin whiskers) in roughly the same places as a man.

But even granting these similarities, the differences (as illustrated in the comparison above) remain striking, even if they are differences in degree rather than in kind. Note that the eye-openings of human beings are much larger than the iris, the colored part of the eye. A large portion of the sclera or white part of the eye is visible. We are the only mammal whose eye-openings are like this. Since human eye brows and eye-lashes are also dramatically visible (particularly on a model in a cosmetic advertisement), we can infer that these features are signal-enhancers. Our eyes enable us to communicate subtle states of mind or emotion at quite a distance, and it is important for us to be able to "read" and interprete each other's moods, attitudes, and impending actions in this way.

Note also that the lips of the model at right seem enlarged compared to the chimp, which has little lip development. Non-verbal sexual messages are very important in human communication, and smiles, along with intense and prolonged eye-contact, constitute an international language. We are, as our faces constantly reveal, very social creatures.

June 16, 1997

Rambling Road to Humanity

Anthropologists debunk another myth of evolutionary progress

By Corey S. Powell and W. Wayt Gibbs

Science has, over the centuries, humbled humans, gradually forcing us to abandon the illusion that our species represents the ultimate end of creation. Copernicus and Galileo displaced Earth from the center of the universe; Darwin dashed the conceit that humans originated in a special way, distinct from all other species. Now a group of researchers--Christopher B. Ruff of Johns Hopkins University, Erik Trinkaus of the University of New Mexico and Trenton W. Holliday at the College of William and Mary--throw cold water even on the notion of steady "improvement" within the human line.

In their recent study, Ruff and his colleagues thoroughly analyzed the fossil record to determine the evolving body mass and brain size of the Homo species leading up to us. The results, published in the May 8 issue of Nature, show just how far from the truth is the stereotypical image of a straight progression from small, pea-brained ancestors to the technologically adept egghead Homo sapiens who inhabit the world today. The truth is quite a bit more complicated.

Hominid brains appear to have remained fairly constant in size for a very long stretch from 1.8 million years ago until about 600,000 years ago--a "period of stasis" whose reality has long been debated by scientists. An abrupt break occurred during the Middle Pleistocene epoch (from 600,000 to 150,000 years before the present), when fossils show that the cranial capacity of our ancestors skyrocketed. This trend peaked roughly 75,000 years ago, when archaic Homo sapiens fossils (a category that includes the well-known Neanderthals) indicate a brain mass of about 1,440 grams. Since then, brain mass has actually drifted downward to the 1,300 grams that is typical today.

Brain size alone does not tell the whole story, of course. Intelligence seems to have less to do with brain size per se than with the brain's proportion to the body it must care for and control (and even that link is rather tenuous). Here, too, the results of the Nature paper are telling. Over the nearly two-million-year span that Ruff and his co-authors examined, ancient hominids were on average about 10 percent more massive than modern humans. Body size peaked about 50,000 years ago: Neanderthals were muscular brutes who weighed upwards of a quarter more than modern humans. Since that time, humans have been marching steadily downhill in both stature and cranial capacity (with the exception of some recent gains due to improved nutrition and reduced disease). The good news is that the steeper decline in body mass over the past 50,000 years has raised our ratio of brain to body above Neanderthal levels, even though total brain mass has dipped.

Calculating the size of our progenitors' brains and bodies from a few scattered bones is a tricky process. Many of the bones vary too much from one individual to another to use for such estimation. Teeth wear differently depending on diet, for example. Eye sockets have changed in proportion over the years, and skulls have grown thinner. Past estimates of the body masses of human ancestors have sometimes disagreed by as much as 50 percent. These disparities made it difficult to assess the changing nature of the human line. But in their methodical survey, Ruff, Trinkaus and Holliday found two variables that appear closely tied to body size in even the most ancient humans: the width of the ball joint on the top of the thighbone (which bears much of our weight when we stand) and the breadth of the pelvis. Measuring these dimensions for 163 fossilized hominids, the scientists were able to plot our genus's changes in brain and brawn.

These improved data are already prompting anthropologists to re-evaluate their assessment of the environmental and cultural transformations that shaped human evolution. In an accompanying commentary in Nature, John Kappelman of the University of Texas at Austin offers some intriguing speculations along these lines. The long, dry spell of constant brain size suggests to him that among our ancestors, as in modern apes, competition among males for access to females may have created an evolutionary pressure favoring continued large bodies. Behavior that was "more dependent on brawn than brains," Kappleman writes, evidently was successful enough that there was little evolutionary pressure toward a bigger cranium.

In considering the new reconstructions of Homo over the past 90,000 years, Kappelman is struck less by the roughly constant brain size than by the rapid decrease in body size, which runs quite counter to the earlier steady or upward trends. He suggests that this decrease in overall bulk was favored "by a social structure that relied on more cooperative foraging and better communication skills." At the same time, a better and more reliable food supply could support the metabolic demands of a large brain. "The increase in relative brain size of modern humans may then be, in part, an effect of selection for smaller body mass," Kappelman rather ignominiously concludes.

So this is what it has come to. The favored son of the Garden of Eden has been demoted to the incredible shrinking human.

August 30, 1999

Old Bones, New Connections

A recently unearthed fossil has scientists rethinking early hominoid evolution

By Kristin Leutwyler

Back in the Middle Miocene era some 16 to 11 million years ago, Earth was planet of the apes. Indeed, so many different hominoids roamed Africa, Europe and Asia then that scientists have had a hard time figuring out from which of these creatures we--and our cousins, the modern apes--are descended. But a recent find, an astonishingly well-preserved partial skeleton from north central Kenya, is helping them prune earlier renditions our hairy evolutionary tree. This skeleton, the most complete ever found from miocene times, is described in a paper in the August 27 issue of Science.

Senior author Steven C. Ward of Northeastern Ohio Universities College of Medicine and project director Andrew Hill of Yale University, along with fossil hunter Boniface Kimeu and researchers from the National Museums of Kenya, found the skeleton, named KNM-TH 28860, in 1993. For a little more than 15 million years, it lay buried uphill from a large number of isolated teeth at Kipsaramon, a site in the Muruyur Formation along the northern crest of the Tugen Hills west of Lake Baringo.

The group moved the specimen in a solid block of soil to the museum, where they pulled out one by one the delicate bones: most of a lower jaw with teeth, upper incisors, bones from the arm, shoulder, collarbone, chest, wrists, fingers and spine. They determined that all of the bones came from a single male.

For six years, they continued to analyze the remains, playing a sophisticated version of the Sesame Street game "one of these things is not like the other." Initially Ward believed the new bones belonged to a middle miocene primate from Kenyapithecus, a controversial genus once thought to be an early ancestor of man and modern apes. But when the researchers took a closer look, they realized that KNM-TH 28860 actually resembled only one of the two known Kenyapithecus species, namely K. africanus and not K. wickeri. They argue in their new paper that because KNM-TH 28860 looks more like K. africanus than K. wickeri does, K. wickeri--and not KNM-TH 28860--is the odd proto-man out.

To remedy the mismatch, they have christened a new genus, Equatorius, for K. africanus and KNM-TH 28860--both of which have been found near the equator. Although this new genus is not a close relative of living apes, the researchers note that it does represent the earliest known ape to spend as much time on the ground as in the trees--a key evolutionary step that our ancestors must have also taken. Ward describes Equatorius as "an animal about the size of a big adult male baboon,...whose arms and legs were about equivalent length, with a long, flexible vertebral column and powerful grasping hands and feet." He adds that they should learn more about its behavior as they continue to study the numerous skeletal bones found.

The genus Kenyapithecus, the research group says, should apply to K. wickeri alone, which is more advanced than the Equitorius lot. This new classification has shored up Kenyapithecus' often shaky status as one of the missing links between middle miocene apes and modern man. Paleontologist Louis Leakey first discovered Kenyapithecus in 1961 at a site called Fort Ternan. He dubbed the 14 million-year old upper jaw and teeth found there K. wickeri. Four years later, he dug up additional teeth and jaw bits on Maboko Island in Lake Victoria, which he called K. africanus.

It was based

on the sophistication of these few teeth that Leakey deemed Kenyapithecus "a very early ancestor of man himself." But later scientists questioned the connection, given the scant fossil record and other seemingly prmitive features of Kenyapithecus. Freeing the genus of its more backward kind, K. africanus, as Ward and his colleagues have done, strengthens the case that Kenyapithecus is in fact our ancient kin.

"One of the reasons for the conflicting opinions about the relationship of Kenyapithecus to the great ape and human clade was that this mixed sample was giving off mixed signals. Different things were thrown into a common pot, muddling up the real story," says co-author Jay Kelley of the University of Illinois. "Kenyapithecus' exact relationship to the origin of these later apes is still somewhat of a mystery at this point, but we think that the work reported in our paper at lest establishes a clearer framework for interpreting how Kenyapithecus might fit into ape and human ancestry."

More support for K. wickeri's ties to modern apes comes from a site called Pasalar in Turkey. It turns out that K. wickeri's dental pattern bears striking similarities to an as yet unclassified 14 million year-old fossil found there, suggesting that Kenyapithecus was one of the few apes that radiated out of Africa early on to other continents. The bones from Pasalar are under further analysis in hopes of clarifying their link to K. wickeri. "The fact that Kenyapithecus can be definitely shown to be more a derived ape with possible affinities to fossils in Turkey opens the door to interesting discussions concerning the ancestry of great apes and humans," Ward comments.

Interesting, for sure. Although Ward and his colleagues put forth a compelling case, not everyone is sold on the idea that Kenyapithecus likely took the first steps out of Africa. David Begun of the University of Toronto calls the new paper extremely important, but guesses that a more prmitive ape along the lines of Equatorius may have made the initial move. He is now preparing a report on a 16 million year-old German fossil ape called Griphopithecus that looks an awful lot like Equatorius. Only more time--or more bones--will tell.

KNM-TH 28860

This partial skeleton, a middle miocene male hominoid from Kenya, offers clues that are helping researchers reclassify some of our earliest possible kin.

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Left mandibular corpus

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Left massillary central incisor (left) and right maxillary lateral incisor (right)

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Right mandibular corpus fragment

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Right scapula

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Right clavicle (right) and left proximal humerus (left)

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Right humerus with first rib attached (left) and right radius (right)

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Right distal ulna (left) and right proximal ulna (right)

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Lowest thoracic vertebra (left) and sternum (right)

October 2001 issue

Finding Homo sapiens' Lost Relatives

Continuing a family tradition, Meave G. Leakey uncovers the skeletons in your closet

By Kate Wong

NAIROBI, KENYA--When Meave Leakey first saw the 3.5-million-year-old human skull, she couldn't help feeling pessimistic. Grass and tree roots had invaded the specimen, and what little of it peeked out through the rocky matrix was riddled with tiny cracks. "It really was a horrible mess," she recalls, an English accent coloring her quiet voice. The veteran paleoanthropologist turns her gray-green gaze from me to the fossil cast sitting on her desk. "I never thought we'd get anything looking as good as this out of it."

This past March, after spending more than a year cleaning and analyzing the skull and a partial upper jaw, unearthed in northern Kenya's Turkana district, Leakey and her colleagues announced that they had assigned the remains to a new hominid genus and species, Kenyanthropus platyops. The fossil possesses a constellation of features--notably a flat face, small teeth and a crest atop its head--that Leakey believes set it entirely apart from the only hominid previously known from that time: Australopithecus afarensis, the species to which the Lucy skeleton belongs. Lucy and her kind have long been considered ancestral to all later hominids--including us--if for no other reason than that A. afarensis appeared to be the only game in town between 3.8 million and three million years ago. If Leakey is correct, however, then at least two hominid lineages existed as far back as 3.5 million years ago. Thus, according to Leakey, it's just as likely that Kenyanthropus--not Australopithecus--gave rise to our own genus, Homo.

Not everyone agrees with her assessment. Paleoanthropologist Tim D. White of the University of California at Berkeley, an expert on early hominids, remains to be convinced that the fossils represent anything but a variant of A. afarensis. Other researchers accept the new species designation but question the new genus. For her part, Leakey notes that time--and more fossils--will tell whether she and her colleagues are right about Kenyanthropus. But she insists that just as later stages of human evolution are characterized by multiple lineages, diversity among early hominids should be expected. Indeed, upending the perception of human evolution as a unilinear progression from quadrupedal ape to upright modern human seems to rank high on Leakey's to-do list.

That a Leakey find has upset a popular view of human evolution is no surprise. In the more than 70 years that the family has searched East Africa for remnants of our past, discoveries that have been made by Louis and Mary Leakey, and later by their son Richard, have forced scholars to revise a number of long-held ideas.

Meave joined the famous family when she married Richard in 1970. Taking over leadership of the annual expeditions to Lake Turkana in 1989, when Richard was appointed head of the Kenya Wildlife Service, she has carried on the family's fossil-hunting tradition ever since. Today, viewed against the site maps and posters of celebrated Leakey fossils adorning the walls of her office at the National Museums of Kenya, Meave seems the very embodiment of her field. So I am somewhat surprised when she reveals that she ended up in it by default.

The eldest of three children, Meave Epps exhibited a keen interest in natural history early on, spending countless hours as a little girl collecting beetles and other insects from the back porch of her family's tiny cottage in Kent, England. She eventually enrolled at the University of North Wales, where she fell in love with marine zoology. But after graduating, a dearth of positions for women in that field led her to consider other options.

Meave's shift to paleontology began when a friend pointed out a job ad on the back page of The Times one afternoon, announcing that Louis Leakey was looking for someone to work at a primate research center in Kenya. Meave raced to the nearest phone booth. She couldn't hear much of what he was saying--she was too busy feeding coins into the phone--but managed to arrange an interview and ended up working for him at the primate center while at the same time doing her Ph.D. research on the forelimb skeleton of modern monkeys.

Meave would soon meet Richard, who had taken over several of his father's many meagerly funded projects while Louis was overseas. Richard was trying to make the finances more manageable, Meave recollects. The first thing he did, she says with a grin, "was call me and say, 'You're spending too much money.'" He later invited her to join the paleontological fieldwork at Lake Turkana. That was 1968; she's worked there ever since.

The early years at Turkana were heady times. "Pretty much every week we were finding a hominid," Meave recalls. Although the tempo of hominid fossil discoveries has slowed since those days--a natural progression considering how little was known and how little had been explored back then--the pace of discoveries about human evolution has not. Under Meave's direction, the fieldwork has become much more focused. Rather than explore new areas, she and her team have revisited previously worked sites, to address specific questions about early hominids.

What prompted our quadrupedal forebears to move from the forests into different environments is one such question. According to evidence Meave and her colleagues have gathered from a site called Lothagam, the evolution of new plants might have played a significant role. Those data indicate that prior to seven million years ago, bushes, trees, shrubs and other plants that use the so-called C3 metabolic pathway dominated the landscape. After that, however, tropical C4 grasses took over--a shift that would have led to the evolution of new grass-eating animals, Meave says, including insects and small vertebrates favored by many primates.

This, in turn, may have set the stage for bipedalism. Standing on two legs, she explains, would have expanded our ancestors' range of gathering when it came to collecting food such as berries, insects and birds' eggs; natural selection favored the giraffe's long neck for the same reason. (Meave is currently preparing several papers relevant to her bipedalism hypothesis.)

Of course, other hypotheses exist. Some propose that two-legged locomotion was more efficient than the quadrupedal variety, others surmise that standing up afforded a better view of potential predators, and still others posit that bipedalism emerged as a way to keep cool, because less of the body is exposed to the sun in an upright position. But as far as Meave is concerned, "they're all fairytales." Moreover, some of these explanations rest on what she believes to be a false notion. "The assumption has always been that our ancestors went straight from the forest into open grassland," she observes. Yet the data indicate that they sometimes occupied more wooded areas.

Meave's own efforts revealed evidence of this when her team found hominid fossils at Kanapoi, another Turkana site, in 1994. These remains and others from nearby Allia Bay revealed a new species she named Australopithecus anamensis. This hominid exhibited clear indications of upright walking, and at 4.1 million years of age, it pushed the earliest evidence of bipedalism--as well as the earliest evidence of the genus--back half a million years. Like A. afarensis, A. anamensis appears to have lived in bushland and open woodland, as indicated by the contemporaneous remains of fauna found at the sites. (Recent discoveries by other researchers have extended the record of two-legged locomotion back further still--to perhaps as many as six million years ago.)

Looking forward, Leakey hopes to uncover additional details of both the bipedalism story and what she considers the next major development in hominid evolution: the emergence of manual dexterity. To that end, she plans to return to the Kenyanthropus site and similarly ancient localities to look for postcranial remains of her new hominid. "If you look at what we knew in 1969 compared to what we know now, it's absolutely incredible. Every month, practically, somebody's found something new," she remarks. "You have no idea which way it's going to go or how it's going to turn out."

Further Information:

Lucy's Legacy: Sex and Intelligence in Human Evolution. Alison Jolly. Harvard University Press, Cambridge, Mass., 1999.

Extinct Humans. Ian Tattersall and Jeffrey H. Schwartz. Westview Press, Boulder, Colo., 2000.

Java Man. Roger Lewin, Garniss Curtis and Carl Swisher. Scribner, 2000.

Adventures in the Bone Trade: The Race to Discover Human Ancestors in Ethiopia's Afar Depression. Jon Kalb. Copernicus Books, 2001.

The Man Who Found the Missing Link: Eugene Dubois and His Thirty-Year Struggle to Prove Darwin Right. Pat Shipman. Simon & Schuster, 2001.

Related Links:

A New Human Ancestor?

New Ethiopian Fossils May Represent Oldest Human Ancestor Yet

New Fossils Reveal Older Human Ancestor

Mary Leakey: Unearthing History

Global Positioning

"Early Hominid Fossils from Africa," by Meave Leakey and Alan Walker (Scientific American, June 1997), is available for purchase at the Scientific American Archive.

December 16, 1996

Mary Leakey: Unearthing History

Editors' Note:
Mary Leakey, one of the world's most renowned hunters of early human fossils, died in Nairobi on December 9, 1996, at the age of 83. Crowning triumphs of her long career included such finds as the 1972 discovery (with Louis, her husband and collaborator) of 1.75-million-year-old remains from Homo habilis at Olduvai Gorge and the 1978 discovery of 3.6-million-year-old footprints at Laetoli, both in Tanzania.

This profile of Dr. Leakey, written by former news editor Marguerite Holloway, originally appeared in the October 1994 issue of Scientific American

By Marguerite Holloway

Mary Leakey waits for my next question, watching from behind a thin curtain of cigar smoke. Leakey is as famous for her precision, her love of strong tobacco--half coronas, preferably Dutch--and her short answers as she is for some of the most significant archaeological and anthropological finds of this century. The latter would have hardly been excavated without her exactitude and toughness. And in a profession scarred by battles of interpretation and of ego, Leakey's unwillingness to speculate about theories of human evolution is unique.

These characteristics have given Leakey a formidable reputation among journalists and some of her colleagues. So have her pets. In her autobiography, Disclosing the Past, Leakey mentions a favorite dog who tended to chomp people whom the archaeologist didn't like, "even if I have given no outward sign." So as we talk in her home outside Nairobi, I sit on the edge of a faded sofa, smiling exuberantly at her two dalmatians, Jenny and Sam, waiting for one of them to bite me. I quickly note details--her father's paintings on the wall, the array of silver trophies from dog shows and a lampshade with cave painting figures on it--in case I have to leave suddenly. But the two dogs and soon a cat and later a puppy sleep or play, and Leakey's answers, while consistently private, seem less terse than simply thoughtful.

Leakey first came to Kenya and Tanzania in 1935 with her husband, the paleontologist Louis Leakey, and except for forays to Europe and the U.S., she has been there ever since. During those many years, she introduced modern archaeological techniques to African fieldwork, using them to unearth stone tools and fossil remains of early humans that have recast the way we view our origins. Her discoveries made the early ape Proconsul, Olduvai Gorge, the skull of Zinjanthropus and the footprints of Laetoli, if not household names, at least terms familiar to many.

Leakey was born in England, raised in large part in France and appears to have been independent, exacting and abhorrent of tradition from her very beginnings. Her father, an artist, took his daughter to see the beautiful cave paintings at such sites as Fond de Gaume and La Mouthe and to view some of the stone and bone tools being studied by French prehistorians. As she has written, these works of art predisposed Leakey toward digging, drawing and early history: "For me it was the sheer instinctive joy of collecting, or indeed one could say treasure hunting: it seemed that this whole area abounded in obje cts of beauty and great intrinsic interest that could be taken from the ground."

These leanings ultimately induced Leakey at the age of about 17 to begin working on archaeological expeditions in the U.K. She also attended lectures on archaeology, prehistory and geology at the London Museum and at University College London. Leakey says she never had the patience for formal education and never attended university; she never attended her governesses either. (At the same time, she is delighted with her many honorary degrees: "Well, I have worked for them by digging in the sun.")

A dinner party following a lecture one evening led her, in turn, to Louis Leakey. In 1934 the renowned researcher asked Mary, already recognized for her artistic talents, to do the illustrations for a book. The two were soon off to East Africa. They made an extraordinary team. "The thing about my mother is that she is very low profile and very hard working," notes Richard E. Leakey, former director of the Kenya Wildlife Service, an iconoclast known for his efforts to ban ivory trading and a distinguished paleontologist. "Her commitment to detail and perfection made my father's career. He would not have been famous without her. She was much more organized and structured and much more of a technician. He was much more excitable, a magician."

What the master and the magician found in their years of brushing away the past did not come easily. From 1935 until 1959 the two worked at various sites throughout Kenya and Tanzania, searching for the elusive remains of early humans. They encountered all kinds of obstacles, including harsh conditions in the bush and sparse funding. Success too was sparse--until 1948. In that year Mary found the first perfectly preserved skull and facial bones of a hominoid, Proconsul, which was about 16 million years old. This tiny Miocene ape, found on Rusinga Island in Lake Victoria, provided anthropologists with their first cranium from what was thought to be the missing link--a tree-dwelling monkey boasting a bigger brain than its contemporaries.

Proconsul was a stupendous find, but it did not improve the flow of funds. The Leakeys remained short of financial support until 1959. The big break came one morning in Olduvai Gorge, an area of Tanzania near the Great Rift Valley that slices East Africa from north to south. Again it was Mary who made the discovery. Louis was sick, and Mary went out to hunt around. Protruding slightly from one of the exposed sections was a roughly 1.8-million-year-old hominid skull, soon dubbed Zinjanthropus. Zinj became the first of a new group--Australopithecus boisei--and the first such skull to be found in East Africa.

"For some reason, that skull caught the imagination," Leakey recalls, pausing now and then to relight her slowly savored cigar or to chastise a dalmatian for being too forward. "But what it also did, and that was very important for our point of view, it caught the imagination of the National Geographic Society, and as a result they funded us for years. That was exciting."

How Zinj fits into the family tree is not something Leakey will speculate about. "I never felt interpretation was my job. What I came to do was to dig things up and take them out as well as I could," she describes. "There is so much we do not know, and the more we do know, the more we realize that early interpretations were completely wrong. It is good mental exercise, but people get so hot and nasty about it, which I think is ridiculous."

I try to press her on another bone of contention: Did we Homo sapiens emerge in Africa, or did we spring up all over the world from different ancestors, a theory referred to as the multiregional hypothesis? Leakey starts to laugh. "You'll get no fun out of me over these things. If I were Richard, I would talk to you for hours about it, but I just don't think it is worth it." She pauses. "I really like to feel that I am on solid ground, and that is never solid ground."

In the field, Leakey was clearly on terra firma. Her sites were carefully plotted and dated, and their stratigraphy--that is, the geologic levels needed to establish the age of finds--was rigorously maintained. In addition to the hominid remains found and catalogued at Olduvai, Leakey discovered tools as old as two million years: Oldowan stone tools. She also recorded how the artifacts changed over time, establishing a second form, Developed Oldowan, that was in use until some 500,000 years ago.

"The archaeological world should be grateful that she was in charge at Olduvai," notes Rick Potts, a physical anthropologist from the Smithsonian Institution who is studying Olorgesailie, a site about an hour south of Nairobi where the Leakeys found ancient stone axes in 1942. Now, as they did then, the tools litter the white, sandy Maasai savanna. The most beautiful ones have been stolen, and one of Leakey's current joys is that the Smithsonian is restoring the site and its small museum and plans to preserve the area.

Olduvai Gorge has not fared as well. After years of residence and work there, and after the death of Louis in 1972, Mary finally retired in 1984. Since then, she has worked to finish a final volume on the Olduvai discoveries and has also written a book on the rock paintings of Tanzania. "I got too old to live in the bush," she explains. "You really need to be youngish and healthy, so it seemed stupid to keep going." Once she left, however, the site was ignored. "I go once a year to the Serengeti to see the wildebeest migrations because that means a lot to me, but I avoid Olduvai if I can because it is a ruin. It is most depressing." In outraged voice, she snaps out a litany of losses: the abandoned site, the ruined museum, the stolen artifacts, the lost catalogues. "Fortunately, there is so much underground still. It is a vast place, and there is plenty more under the surface for future generations that are better educated."

Leakey's most dramatic discovery, made in 1978, and the one that she considers most important, has also been all but destroyed since she left the field. The footprints of Laetoli, an area near Olduvai, gave the world the first positive evidence of bipedalism. Three hominids (generally identified as Australopithecus afarensis ) had walked over volcanic ash, which fossilized, preserving their tracks. The terrain was found to be about 3.6 million years old. Although there had been suggestions in the leg bones of other hominid fossils, the footprints made the age of bipedalism incontrovertible. "It was not as exciting as some of the other discoveries, because we did not know what we had," she notes. "Of course, when we realized what they were, then it was really exciting."

Today the famous footprints may only be salvaged with the intervention of the Getty Conservation Institute. "Oh, they are in a terrible state," Leakey exclaims. "When I left, I covered them over with a mound of river sand and then some plastic sheeting and then more sand and a lot of boulders on top to keep the animals off and the Maasai off." But acacia trees took root and grew down among the tracks and broke them up.

Although Leakey steers clear of controversy in her answers and her writings, she has not entirely escaped it. She and Donald Johanson, a paleontologis t at the Institute of Human Origins in Berkeley, Calif., have feuded about the relation between early humans found in Ethiopia and in Laetoli. (Johanson set up his organization as a philosophical counterweight to the L.S.B. Leakey Foundation.) And some debate erupted about how many prints there were at Laetoli. Tim White of the University of California at Berkeley claimed that there were only two and that Leakey and her crew had made the other track with a tool during excavation. Leakey's response? "It was a nonsense," she laughs, and we are on to the next subject.

A subject Leakey does not like. "'What was it like to be a woman? A mother? A wife?' I mean that is all such nonsense," she declares. Leakey--like many other female scientists of her generation, including Nobel laureates Rita Levi-Montalcini and Gertrude Belle Elion--dislikes questions about being a woman in a man's field. Her sex played no role in her work, she asserts. She just did what she wanted to do. "I was never conscious of it. I am not lying for the sake of anything. I never felt disadvantaged."

Leakey just did her work, surviving bitter professional wars in anthropolog y and political upheavals. In 1952 Louis, who had been made a member of the Kikuyu tribe during his childhood in Africa, was marked for death during the Mau Mau uprising. The four years during the height of the rebellion were terrifying for the country. The brakes on Mary's car were tampered with, and a relative of Louis's was murdered. The house that Leakey lives in today was designed during this time: a low, white square structure with a central courtyard where the dogs can run at night.

These pets are very important to Leakey--a source of companionship and safety out in the bush. She admires the traits in them that others admire in her: independence and initiative. (Any small joy that I have about emerging from her house unbitten fades sadly when I reread the section in her autobiography about her telepathic dalmatian and learn that he died years ago.)

We seem to have covered everything, and so she reviews her discoveries aloud. "But you have not mentioned the fruits," she reminds me. One of Leakey's favorite finds is an assortment of Miocene fossils: intact fruits, seeds, insects--including one entire ant nest--and a lizard with its tongue hanging out. They lay all over the sandy ground of Rusinga Island. "We only found them because we sat down to smoke a cigarette, hot and tired, and just saw all these fruits lying on the ground next to us. Before that we had been walking all over them all over the place." She stops. "You know, you only find what you are looking for, really, if the truth be known."

December 05, 2000

New Fossils Reveal Older Human Ancestor

Scientists working 150 miles northeast of Nairobi in Kenya’s Baringo district have discovered the remains of what may be the earliest human ancestor known. At a news conference yesterday, the team reported that the bones date to at least six millions years ago, which makes them more than 1.5 million years older than Ethiopian fossils previously held to represent the earliest hominid. The French and Kenyan researchers found the first remains of the new hominid, nicknamed Millennium Man, in late October. Since then they have unearthed bones belonging to at least five individuals. "Not only is this find older than any else previously known," team member Martin Pickford of the College of France said, according to a Reuters report, "it is also in a more advanced stage of evolution." Among the new hominid’s advanced features are its upper leg bones, which suggest that the chimpanzee-sized Millennium Man walked upright, and its dentition: its small canines and large molars are similar to the modern condition. Exactly how this proto-human relates to other ancient members of the hominid family, however, remains to be seen. For now the team plans to publish its initial findings, and push ahead with excavation. "I am sure there is still a lot more out there," Pickford said, "possibly even older." --Kate Wong

March 22, 2001

A New Human Ancestor?

Researchers working in Kenya have unearthed fossils that could upset a long-held view of human origins. According to a report published today in the journal Nature, the remains represent a previously unknown hominid genus and species from which modern humans may have descended. For decades that distinction belonged to Australopithecus afarensis—a species typified by the famous 3.2-million-year-old Lucy skeleton. But the new discovery, dubbed Kenyanthropus platyops, comes from roughly the same time interval and thus renders Lucy's ancestral status far less certain.

Meave Leakey of the National Museums of Kenya and her colleagues recovered the Kenyanthropus remains—which include jaws, teeth and a skull dated to between 3.2 and 3.5 million years old—from mudstone sediments in northern Kenya’s Turkana basin. (The skull is apparently the oldest almost complete early human cranium known.) Subsequent analysis revealed a creature rather different from Lucy and her kin. In particular, Kenyanthropus has a far flatter face and smaller molar teeth—feeding-related differences that suggest the two groups could have coexisted without competing for food resources.

"Now that we have a new form of early hominid from the same time period that is quite distinct from afarensis, the anthropologists will have to decide which of these forms of early human actually lies in our ancestral tree," University of Utah geologist Frank Brown, who dated the fossil, remarks. "It cannot be both." --Kate Wong

July 12, 2001

New Ethiopian Fossils May Represent Oldest Human Ancestor Yet

Researchers working in Ethiopia have unearthed the remains of a creature thought to occupy a spot on the family tree quite close the evolutionary branching point between humans and chimpanzees. According to a report published today in the journal Nature, these remains belong to a new subspecies of the hominid genus Ardipithecus, previously known fossils of which date to 4.4 million years ago. The new fossils, however, come from sediments dated to between 5.2 and 5.8 million years ago, and thus may represent the earliest known human ancestor.

Yohannes Haile-Selassie, a doctoral candidate at the University of California at Berkeley, discovered the fossils in Ethiopia's Middle Awash study area, a locale famous for yielding other key fossils, such as the 4.4 million-year-old Ardipithecus ramidus and the 2.5 million-year-old Australopithecus garhi. Subsequent analyses of the fragmentary remains—which include a jawbone with teeth; hand, foot and arm bones; and a piece of collar bone—revealed a bipedal creature with teeth similar to those of later hominids. These teeth and certain skeletal features are more primitive than those belonging to Ardipithecus ramidus, however, thus leading Haile-Selassie to designate the fossils as a new subspecies, Ardipithecus ramidus kadabba.

A.r.kadabba is not without competition for the distinction of being the earliest human ancestor. Earlier this year a French team unveiled the 6 million-year-old remains of Orrorin tugenensis, a new species that the discoverers regard as hominid. (These researchers consider Ardipithecus a chimpanzee ancestor.) For his part, Haile-Selassie asserts that more information is required to resolve Orrorin's place in the family tree. And with excavations resuming in the Middle Awash this fall, more clues to our elusive evolutionary past may yet come from Ardipithecus. --Kate Wong

January 2003 issue

PALEOANTHROPOLOGY

An Ancestor to Call Our Own

Controversial new fossils could bring scientists closer than ever to the origin of humanity

By Kate Wong

POITIERS, FRANCE--Michel Brunet removes the cracked, brown skull from its padlocked, foam-lined metal carrying case and carefully places it on the desk in front of me. It is about the size of a coconut, with a slight snout and a thick brow visoring its stony sockets. To my inexpert eye, the face is at once foreign and inscrutably familiar. To Brunet, a paleontologist at the University of Poitiers, it is the visage of the lost relative he has sought for 26 years. "He is the oldest one," the veteran fossil hunter murmurs, "the oldest hominid."

Brunet and his team set the field of paleoanthropology abuzz when they unveiled their find last July. Unearthed from sandstorm-scoured deposits in northern Chad's Djurab Desert, the astonishingly complete cranium-- dubbed Sahelanthropus tchadensis (and nicknamed Toumaп, which means "hope of life" in the local Goran language)-- dates to nearly seven million years ago. It may thus represent the earliest human forebear on record, one who Brunet says "could touch with his finger" the point at which our lineage and the one leading to our closest living relative, the chimpanzee, diverged....

July 08, 2002

First Humans to Leave Africa Weren't Necessarily a Brainy Bunch

A longstanding view of human evolution holds that the first hominids to leave Africa did so with the help of bigger brains, longer legs and fancier tools than those of their predecessors. That scenario suffered a major blow a couple of years ago, however, when paleontologists working in Dmanisi, Georgia unearthed the oldest human remains yet found outside of Africa---two 1.7-million-year-old skulls belonging to early members of our genus, Homo--and discovered primitive tools alongside them. Now a new finding may topple another pillar of the theory. According to a report in the current issue of the journal Science, researchers working at the same site have recovered a third skull--one that housed a surprisingly small brain.

Exceptionally well-preserved, the fossil, dubbed D2700, exhibits a thin browridge, short nose and large canine teeth, observes team member David Lordkipanidze of the Georgian Academy of Sciences in Tbilisi. With an endocranial volume of roughly 600 cubic centimeters, D2700 was considerably smaller-brained than the other two Dmanisi hominids, whose skulls have some 800 cubic centimeters of brain space. But size differences notwithstanding, the three specimens are, overall, similar in form. The investigators thus believe that the remains represent members of the same species, H. erectus, although they have some features characteristic of the more primitive Homo habilis. It may well be that the D2700 individual was a female and the other two were males, the team posits.

Only time and more fossils will reveal who these intrepid travelers were and why they left their motherland. But as is so often the case in paleoanthropology, the emerging picture appears to be far more complex than previously thought. --Kate Wong

June 11, 2001

The Dating Game

Geologists, paleontologists and archaeologists have pieced together a fairly detailed account of how Earth and its inhabitants evolved. But just how do researchers determine the ages of the materials they unearth? (Hint: It’s not just carbon-dating anymore.)

These days hardly a week goes by without important discoveries concerning the history of life on Earth making headlines. Indeed, just last month researchers described a fossil that pushes the origins of key mammal features back some 45 million years. And last week scientists announced that new dates for an extinction event that claimed most of Australia's large animals show that humans, not the climate, wiped them out. Although visual inspection of the rocks, fossils and archaeological remains used to reconstruct our planet's past provides critical information, only by ascertaining their ages can researchers put this data into a meaningful context.

The first step toward accurately measuring geologic time came at the turn of the 20th century, when French physicist Henry Becquerel discovered the natural radioactive decay of uranium. Shortly thereafter, building on related work by Ernest Rutherford, American chemist Bertram Borden Boltwood determined that he could use the predictable decay of radioactive elements such as uranium into other elements to keep track of time. Although Boltwood's resulting estimates for things like the age of Earth—which he placed at around 2.2 billion years—have since been significantly revised, he indicated correctly that our planet was far older than people had imagined possible.

In the decades that followed, scientists made important new discoveries about the structure and behavior of atoms, and they refined their existing dating techniques. More recently, they have developed a number of new methods. Some use radioactive isotopes; others take advantage of different phenomena, such as thermoluminescence and electron spin resonance. Still others, like amino acid racemization, show promise but have not yet taken wing.

Now, nearly 100 years after Boltwood's groundbreaking work, it is estimated that Earth formed at least twice as long ago as he had claimed. The following summaries offer a quick introduction to some of the dating techniques researchers have been using to explore and reconstruct our planet's past, from 4.5 billion years ago to the present. —Kate Wong
 

RADIOACTIVE ISOTOPES

The premise behind techniques involving the use of radioactive isotopes is straightforward. Each isotope has what is known as a half-life—that is, a period of time in which half of the atoms in a population decay into stable daughter elements. This half-life differs dramatically from isotope to isotope. As a result, different isotopes are better suited to dating different items. In the case of carbon 14, for example, the half life is only 5,730 years. Carbon 14 can thus reliably date items only up to around 40,000 years old.

Other radioactive isotopes can be used to accurately date objects far older. The decay of argon 40 to argon 39, for instance, played a vital role in underscoring the significance of two ancient human skulls unearthed in the Republic of Georgia last summer. These remains, Carl C. Swisher III of the Berkeley Geochronology Center and his colleagues reported, are more than 1.7 million years old, and as such represent the first humans to leave Africa to colonize the rest of the world. Argon dating can also be used to date materials as young as 10,000 years and as old as billions of years.

Uranium and lead isotopes take us back farther still. Indeed, findings presented earlier this year suggest that infant Earth may have been ready to support life far earlier than previously thought. Uranium-lead dates for a single zircon crystal found in the oldest sedimentary rock yet known suggest that by 4.4 billion years ago our planet already had already cooled enough to have a crust. The first life-forms may have been just around the corner.
 

THERMOLUMINESCENCE AND ELECTRON SPIN RESONANCE

Many crystals, including diamond, quartz and feldspar, accumulate and trap electric charges at a known rate over time. Heating the crystals, it turns out, liberates these electrons, emitting a measurable amount of light. Researchers can thus determine the amount of time that has passed since the buried crystal was last exposed to heat.

In the case of thermoluminescence, resetting the crystal clock means heating it to around 500 degrees Celsius. Because of that condition, scientists say, the technique is well suited to dating meteoritic impacts, fire-treated stones used by early humans, cooking hearths and old ceramics.

Somewhat similar to thermoluminescence, electron spin resonance (ESR) dates crystals, too (although these are found in shells and enamel.) Unlike thermoluminescence, however, this method counts the number of "unpaired spins" of electrons trapped in the crystal, instead of freeing them.

ESR can be used to evaluate materials up to one million years old and has become an indispensable tool for paleoanthropologists, who often use it to date the teeth of animal remains found among the precious human fossils.
 

AMINO ACID RACEMIZATION

Over time, the amino acids that make up proteins slowly convert from their so-called left-handed state to their right-handed form—a phenomenon known as racemization. When temperature and environment are constant, conversion occurs at a constant rate.

In theory, this should allow researchers to date protein up to 100,000 years old. So far, however, the technique has proved problematic—perhaps because it is difficult to know whether conditions have been constant. (Some researchers have suggested, though, that levels of amino acid racemization can be good indicators of ancient DNA preservation.)