Is The Eastern Lowland Gorilla A Hybrid?

Thursday, December 17, 2009

Hybrids, hominoids and hominins

Is the Eastern lowland gorilla (Gorilla beringei graueri) a hybrid (sub)species? A recent study by RR Ackermann and JM Bishop suggests this scenario.

Their study used morphological, genetic and geographic information to analyze variation in extant gorilla species and subspecies. A previous study by Ackermann and colleagues (2006) on baboons found that non-metric traits--namely pairs of extra teeth and unusual sutures between some facial bones--have freakishly high frequencies in known hybrids compared to their parents of different species. Well wouldn't you know it: G. b. graueri had a significantly higher frequency of such traits than the other gorilla species/subspecies (the putative 'parental' species, the Eastern mountain gorilla G. b. beringei and the Western lowland gorilla G. gorilla gorilla).

Additionally, for a number of cranial metric traits, graueri had significantly higher values than the other gorilla species' sample averages. "Heterosis" refers to a condition wherein a hybrid phenotype exceeds the combined parental mean--it appears that if this is truly a hybrid population, graueri displays heterosis for a number of cranial features. Finally, it is notable that graueri has been described as more like Western gorillas in some respects, but more like Eastern mountain gorillas in others, and then totally unique in some aspects. This is arguably a result of graueri possessing genes from two other distinct species.

Oh, and the mtDNA evidence suggests fairly recent gene flow from Western lowland gorillas eastward. Because mtDNA is maternally inherited, this implies that females have been involved in this west-to-east gene flow. However, it is unclear the extent to which there was east-to-west gene flow, or the potential involvement of male gorillas here.

Why is graueri likely a largely hybrid sample, and not just part of a morphological and genetic cline conecting Western and Eastern gorilla populations (i.e. making gorillas a single, polymorphic, geographically broad species)? The hybrid morphologies above are believed to indicate complications that arise in development, due to the union of two species' distinct sets of genes. Such signatures of hybridizaiton would not be expected to appear in a regularly panmictic species. It also seems that the separation of Western and Eastern gorilla species occurred during the Pleistocene, which means that the two sides have been diverging for quite a long time and have recently come back into contact.

I think the authors make a very good case for the importance of hybridization in the evoluton of gorillas, at least as we know them today. I like their use of both morphological and genetic data, which complement one another nicely in support of a hybrid-type nature of Gorilla beringei graueri. In addition, the implicaitons of the study are fantastic! Even though the authors did not know for sure whether individual specimens were hybrids, they were able to use the results of previous work to make a convicning case that a number of their specimens were very likely to be hybrids. This is a good sign for persons like myself who are interested in the detection of hybrids in skeletal/fossil samples. Another great implication is that hybridization indeed has a place in hominoid evolution--it awaits to be seen what role hybridization may have played in the course of human evolution.

References
Ackermann RR, Rogers J and Cheverud JM. 2006. Identifying the morphological signatures of hybridization in primate and human evolution. Journal of Human Evolution 51: 621-645.

Ackermann RR and JM Bishop. Morphological and molecular evidence reveals recent hybridization between gorilla taxa. Evolution: in press.

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Is Homo Sapiens Polytypic?

An interesting paper worth reading, which considers the idea that Homo sapiens can be subdivided to subspecies against two diametrically opposite ideas, namely (i) that there are no human subspecies, and (ii) that human taxonomic differences warrant the rank of species. The author rejects (i) on the grounds that Homo sapiens exhibit higher levels of diversity (in terms of heterozygosity and Fst) compared to species where subspecies are recognized. I had not heard of (ii) argued recently, but Woodley cites Fuerle as a recent supporter, offering the following criticism:

FST reflects the relative amount of total genetic differentiation between populations, however different measures of genetic distance involving mtDNA and autosomal loci are simply inappropriate for the purposes of inter-specific comparison as the different genes involved will have been subject to markedly different selection pressures and are therefore not likely to have diverged at the same time [62]. To illustrate this point, this author listed alternative estimates of the distance between the gorilla species and the common chimpanzee and bonobo, based on various nuclear loci and autosomal DNA. The much higher numbers reflect the extreme variation that can be expected when different genes are considered. Fuerle’s presentation of the data is also problematic for another reason, namely he makes no mention of the current debates surrounding gorilla and chimpanzee/bonobo taxonomy; as new research on these taxa regularly generates novel and in some cases wildly variable estimates of genetic distance between these primates, and there is even some debate over whether the eastern and western gorillas are separate species [60].

Curnoe and Thorne have estimated that periods of around two million years were required for the production of sufficient genetic distances to represent speciation within the human ancestral lineage [56]. This indicates that the genetic distances between the races are too small to warrant differentiation at the level of biological species, as the evolution of racial variation within H. sapiens started to occur only 60,000 years ago, when the ancestors of modern humans first left Africa.

Personally I think that the evidence is clear that human races or subspecies exist, but the discovery that geographic differentiation exists at the level of races, ethnic groups, sub-ethnic groups, and that even villages can be subdivided into geographically distinguishable clusters, make renewed effort into formalizing taxonomy at the sub-species level an especially worthwhile endeavor.

Medical Hypotheses doi:10.1016/j.mehy.2009.07.046

Is Homo sapiens polytypic? Human taxonomic diversity and its implications

Michael A. Woodley

Abstract

The term race is a traditional synonym for subspecies, however it is frequently asserted that Homo sapiens is monotypic and that what are termed races are nothing more than biological illusions. In this manuscript a case is made for the hypothesis that H. sapiens is polytypic, and in this way is no different from other species exhibiting similar levels of genetic and morphological diversity. First it is demonstrated that the four major definitions of race/subspecies can be shown to be synonymous within the context of the framework of race as a correlation structure of traits. Next the issue of taxonomic classification is considered where it is demonstrated that H. sapiens possesses high levels morphological diversity, genetic heterozygosity and differentiation (FST) compared to many species that are acknowledged to be polytypic with respect to subspecies. Racial variation is then evaluated in light of the phylogenetic species concept, where it is suggested that the least inclusive monophyletic units exist below the level of species within H. sapiens indicating the existence of a number of potential human phylogenetic species; and the biological species concept, where it is determined that racial variation is too small to represent differentiation at the level of biological species. Finally the implications of this are discussed in the context of anthropology where an accurate picture of the sequence and timing of events during the evolution of human taxa are required for a complete picture of human evolution, and medicine, where a greater appreciation of the role played by human taxonomic differences in disease susceptibility and treatment responsiveness will save lives in the future.

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Does a Better Diet Equal a Larger Brain

March 31, 2009, Evolutionary Psychology

Surprisinly, brain size linked to diet rather than clever handiwork

Despite having large brains our ancestors left few signs of intelligent behavior. A million years ago, their tools were primitive and unvaried. Many anthropologists now believe our oversized noggins were the result of improved diet, rather than fancy craftwork.

The role of diet in brain size comes into sharper focus by considering another explanation for our large brains, the fact that humans mature more slowly than other primates.

Do large-brained adults preserve the large-brained trait of juveniles
Ever notice that toddlers have disproportionately large heads making them top heavy and liable to fall on their heads? What if the infantile trait of a relatively large head were preserved into maturity (a process called neoteny)? One reason this happens is when development is slowed to prolong the juvenile period. Slowing development can be an advantage if it increases body size, or if it permits us to learn more.

The clearest evidence for neoteny in head shape is the fact that adult humans have a marked resemblance to young chimps in the proportions of the head and face. This phenomenon has impressed comparative anatomists at least since the time of Darwin. Our heads look too big for an adult primate, but not for a juvenile.

The neoteny hypothesis is intriguing but suffers from a fatal weakness. Brains are terrifically expensive to run: they require ten times as much energy per pound as the rest of the body. The neoteny theory cannot explain why humans would preserve such a large energetically expensive brain. Under this scenario, slightly smaller brains would be selected for over many generations until the brain was no larger than it needed to be. This weakness is tackled head on by the dietary theory, known as the expensive tissue hypothesis (1).

The expensive tissue hypothesis
The expensive tissue hypothesis is rather like the chalk line drawn around a homicide victim - everything gets reduced to a single dimension: dietary energy. Taking the body as a whole, two tissues use up a lot of energy, the gut and the brain. When humans are compared to a typical primate, most tissue types use about the same amount of energy with two striking exceptions. The human gut (intestines) uses less energy than a typical primate and the human brain uses far more.

What really excites physical anthropologists is the fact that the energy subtracted from the gut's energy budget gets added, almost exactly to the brain's budget. What can this mean? Evidently brain size is normally held in check by its great energy cost. For humans, though, reduction in the size of the gut created "surplus" energy that permitted an increase in the size of the brain.

So why did the human gut get reduced? Our ancestors switched to a more refined diet that required both less chewing and less digesting. Greater consumption of meat is a plausible explanation here. It is no accident that hunting mammals such as lions and African hunting dogs have comparatively large brains and comparatively small guts. It all fits together like the body lying inside its chalk line.

One of the hardest aspects of human brain evolution to explain is why it has been so rapid, almost doubling in a million years. It is as though brain size were promoting itself like a wild fire rushing through a forest. Why might brain expansion feed itself? When brains get bigger, and humans get smarter, they are more successful at figuring out how to trap game animals, so the proportion of refined food increases and the size of the gut declines. Energetically speaking, this makes room for further expansion in brain size. This is positive feedback.

If it were a car, we might say that the expensive tissue hypothesis was a sweet ride. Yet, it cannot go everywhere or do everything. In particular, it doesn't explain why humans suddenly began getting so much smarter than other primates some 200,000 years ago. This enigma is taken up in another post.

1. Aiello, L. C., & Wheeler, P. (1995). The expensive tissue hypothesis. Current Anthropology, 36, 199-221.

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Primatologist Brian Hare Investigates Social Behaviour in Chimpanzees and Bonobos

When Brian Hare started college in 1994, he apprenticed himself to a top chimpanzee researcher. His mentor, Michael Tomasello, was just beginning to investigate whether chimpanzees can understand what another chimp—or perhaps even a human—is thinking. Hare said such a skill couldn't be that difficult. After all, he told Tomasello, "my dog does that." Tomasello looked skeptical. And then he spoke the words that often inspire scientific discovery: "Prove it.
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Genetic Difference, Wild VS Tame Animals

A study of nasty and nice lab rats has scientists on the verge of knowing the genes that separate wild animals like lions and wolves from their tame cousins, cats and dogs.

Unlike their wild ancestors, house pets and other domesticated animals share the trait of tameness, meaning they tolerate or even seek out human presence. New research, which is published in the June issue of the journal Genetics and involved the interbreeding of friendly and aggressive rats, reveals gene regions that influence the opposing behaviors.

"I hope our study will ultimately lead to a detailed understanding of the genetics and biology of tameness," said researcher Frank Albert of the Max Planck Institute for Evolutionary Anthropology in Germany. "Maybe we'll then be able to domesticate a few of those species where humans have historically not been successful like the wild African Buffalo."

And we can possibly understand more about the furry creatures in our homes.

"If you think about dogs, they are such amazing animals. When you compare a dog with a wild wolf, a wolf has no interest in communicating [with] or tolerating humans," Albert told LiveScience. "If you're lucky a wolf in the wild wouldn't care about you. But a dog does care and they even seek human presence."

He added, "Dogs were all wolves at some point. How did they become these animals that need humans to exist?"

Breeding rats

The roots of this study date back to 1972 when researchers in Novosibirsk, in what is now Russia, caught a large group of wild rats around the city. Back at the lab, the researchers arbitrarily separated the rats into two groups. In one group, called the tame rats, the scientists then mated the friendliest rats, those that tolerated humans, with one another, and in the other group they mated the most aggressive rats with each other.

Demeanor in rats is tested with the glove test, in which a human hand protected by a metal glove approaches a caged rat. The tame rats tolerate the hand and even sometimes toddle across it. Aggressive rats try to escape, scream, attack and bite the person's hand. The rats even perform boxing moves, standing on their hind legs while sort of punching the human hand away.

The experiment is going on to this day, with two generations bred each year, resulting in a team of extremely tame rats and a team of very aggressive ones.

Nice genes

To figure out the genes behind the rat behaviors, Albert and his colleagues interbred a few of the tamest rats with a few of the aggressive rats and then interbred the resulting pups. That way, the rats would have a mix of genes from both types of parents.

So if two rats had matching genes in one region of their genomes but differing tameness behaviors, the researchers could rule out this genetic region as responsible for the behaviors. The inverse is also true.

First, behavior tests teased out which rats were naughty and which were nice. Then, the researchers ran genetic tests. While the results don't reveal specific tameness genes, the researchers have pinpointed sets of genes responsible for tameness.

Further breeding and testing will hopefully uncover the exact genes linked with certain rat behaviors.

• Evolution's Most Extreme Mammals
• Dog News, Information and Images
• House Cats' Wild Ancestor Found

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Charles Darwin's Egg, 200 YO

An egg collected by naturalist Charles Darwin on his
HMS Beagle voyage which has been rediscovered at a museum.
Photo: PA

Researchers have known that the naturalist collected 16 bird eggs during his trip between 1831 and 1836 but all were thought to be lost.

But one sample – that of the Tinamou bird of Uruguay - has been discovered by a volunteer as she catalogued a collection at the Zoology Museum.

The records seem to indicate that Darwin himself was responsible for damage caused to the heavily cracked egg after packing it in too small a box during or following his famous voyage.

The chocolate brown egg – slightly smaller than a hen's egg – was among the museum's 10,000 strong collection from Darwin being partly catalogued by volunteer Liz Wetton.

She has spent half a day at the Museum each week for the past ten years where she faithfully sorts and reboxes the Museum's bird egg collection.

She merely commented that the specimen had C. Darwin written on it before moving to the next drawer.

It was only when Mathew Lowe, Collections Manager, was reviewing her work that he discovered no one knew about the existence of this specimen.

He said: "There are so many historical treasures in the collection, Liz did not realise this was a new discovery. To have rediscovered a Beagle specimen in the 200th year of Darwin's birth is special enough, but to have evidence that Darwin himself broke it is a wonderful twist."

After reading Liz's notes, Lowe and Curator of Ornithology Dr Mike Brooke, traced the specimen's origin in the notebook of Professor Alfred Newton, a friend of Darwin's and Professor of Zoology in the latter 19th Century.

Prof Newton had written: "One egg, received through Frank Darwin, having been sent to me by his father who said he got it at Maldonado (Uruguay) and that it belonged to the Common Tinamou of those parts.

"The great man put it into too small a box and hence its unhappy state."

Museum Director Professor Michael Akam said: "This find shows just how valuable the work of our loyal volunteers is to the Museum. Only Liz has examined each of the many thousands of eggs in our collection. Without her we would not have found this unique specimen."

Ms Wetton said: "It was an exhilarating experience. After working on the egg collections for ten years this was a tremendous thing to happen.""

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Anthropologists, The hobbit

Sydney - The arguments go on and on over a pile of bones found in a cave on the Indonesian island of Flores five years ago. Some anthropologists are insistent that the skeleton of a metre-tall woman who hunted pygmy elephants and giant rats 18,000 years ago was that of a modern human with a deformed brain rather than a remnant of a separate species of human that died out. Dwarfism, they say, came about because they lived on a small island and their brains, and then their bodies, shrank to accord with the limited demands of their environment. Others find this theory implausible. Mike Morwood, the University of Wollongong anthropology professor who co-led the Flores team with the late Professor Soejono of the Indonesian Centre for Archaeology, argues that recent research bolsters the case that Homo floresensis, the primitive human dubbed the hobbit, was indeed a branch off human evolution that ended up going nowhere. "They are the wrong species in the wrong place at the wrong time," Morwood said. The research, published in the journal Nature and the work of Professor William Jungers of New York's Stony Brook University Medical Centre, focuses on the hobbit's foot. It's flat, relatively long, good for walking but not for running ñ and akin to those of ancient humans who lived in Africa more than 3 million years ago. Morwood, Jungers and many other anthropology luminaries, argue that the foot clinches it: the hobbit could at a pinch be a victim of dwarfism but its primitive foot showed the overwhelmingly likely explanation is that it's a separate species of primitive human that died out."

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Prehistoric Cave Art

BUTTE VALLEY -- What would inspire prehistoric people to crawl deep down into dark caves just to draw? As in the case of Lascaux in southwestern France, nobody really knows for sure. The only thing certain is that we are left with a beautiful mystery.

The French caves are home of some of the most famous Upper Paleolithic art (roughly 38,000 to 15,000 years ago) which depicts images of cattle, bison, horses, bears, felines, birds and even rhinoceros. The drawings were randomly discovered in 1940 and have been a sight of wonder and curiosity ever since.

According to Butte College Anthropology instructor, Mike Findlay, the non-utilitarian function of the cave art most likely connects to some form of animism for the early hunter-gatherers of the region. Fascinated by the obscurity, Findlay seized the opportunity -- and an empty cement retaining wall -- to bring the Lascaux paintings and mystery to life in Butte County in 2006. Now, the replica murals serve as educational campus décor.

"I bring some of my honors and physical anthropology classes out (to the paintings) and ask them: 'Why would they do this?' It gives us a chance to brainstorm theories," he said, and to bring prehistoric art and culture to life. The International Committee for the Preservation of Lascaux states the artists who created the paintings lived in Europe during the finals years of the last Ice Age, approximately 17,000 years ago.

A mixture of earth minerals like iron oxides (ochres) were typically used in prehistoric art to produce reds, yellows, oranges and browns or manganese for blacks and grays, while prehistoric "brushes" consisted of animal or human hair, fur, and plant particles. When Findlay led fellow Butte College staff and students in reproducing the cave paintings three years ago, he opted for modern acrylics and tools. All together the team drew four Lascaux images, two European bison from Alta Mira, Spain, and five Chauvet cave paintings from southern France. The art now graces the retaining wall facing the Learning Resource Center on the main Butte College campus.

"There has been a positive response to the murals," Findlay said. "I don't think everyone is familiar with Upper Paleolithic art," he said with a laugh, "but they appreciate the warm colors."

Allison Ehresman, a Butte College student who participated in the cave art project recounted, "It was a fun thing to do. I love paint and art, and I wanted to express my love for anthropology." Ehresman also expressed satisfaction for leaving a legacy on campus that will continue providing mystery and beauty for others, just like Lascaux has, for years to come."

DeeAnn Resk can be contacted at deeannresk@gmail.com.

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Tree Climbing Abilities of Early Hominins Decreased

Anthropologist Says Tree Climbing Abilities of Early Hominins Decreased Rapidly in Evolutionary Process

April 15th, 2009 by Mary Anne Simpson

Credit: Mongabay.com

Jeremy M. DeSilva an anthropologist at Worcester University in Massachusetts has published "Functional Morphology of the Ankle and the Likelihood of Climbing in Early Hominins," in the peer-reviewed journal, Proceeding of the National Academies of Sciences of the USA current issue. The study includes data gathered by DeSilva in Uganda's Kibale National Park of modern chimpanzee and comparisons of hominin fossil skeletal remains dating back some 4.12 million to 1.53 million years ago. The findings appear to show that if early hominins depended on tree climbing as part of their survival repertoire, they were performing it decidedly different from modern chimpanzee locomotor activity.

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Most human evolution researchers agree that at some point in history, estimated to be about 5 million to 7 million years ago the chimpanzee and human like species proceeded on a different evolutionary path. The question DeSilva addresses is whether early man's adaptation to full bipedalism involved a swift shedding of the ability to climb and swing from trees. DeSilva compared the great apes and early hominin ankle joint, the tibia and the talus in the foot. He discovered marked differences between the structure and capacity of these two skeletal fossils.

DeSilva observed and filmed Uganda's modern chimps tree climbing and tree swinging activity. He discovered that modern chimpanzees when pushing off from a tree branch flex their ankles, thereby raising their foot some 45 degrees. Modern man generally flexes his ankle only 15 to 20 degrees while walking and suffers injury if stretched any further. It goes without saying, there are certain super modern athletes who can flex the ankle without injury.

In effect, DeSilva points to an important evolutionary divide, wherein early hominin may have foregone his natural tree climbing, ankle flexing capacity in order to walk upright. His examination of early hominin and the great ape indicates the evolutionary morphing took place very rapidly. His review of early hominin and the great ape demonstrate the ankle joint differences took place early in the evolutionary cycle.

According to ScienceNow reporter Michael Baiter in his article, "Our Ancestors Were No Swingers", David Begum, a paleoanthropologist at the University of Toronto in Canada points to early hominins ability to scramble up a tree for safety which would not have involved or required the full 45 degree ankle flex exhibited by modern chimpanzees.

The DeSilva study agrees with its critics. He concludes that if early hominins included tree climbing as part of their survival skill set, they were performing it very differently than modern chimpanzees."

Sources:

PNAS, April 13, 2009, http://www.pnas.org/content/early/2009/04/13/0900270106.abstract
Science Now, April 13, 2009, http://sciencenow.sciencemag.org/cgi/content/full/2009/413/2

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Top 10 Missing Links

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Caveman VS. Modern Man

Olympic athletes may benefit from today's sports drinks and high-tech training, but their gymnastics or wrestling performance probably pales in comparison to what early human ancestors could have pulled off.

That's because we Homo sapiens have followed an evolutionary track away from sheer body strength and toward the lean, mean endurance qualities of a long-distance runner.

"The chimp-like ancestor was like a power athlete," said Dan Lieberman, a biological anthropologist at Harvard University. "Much stronger and faster than humans, but they had no endurance."

Neanderthals, who coexisted with Homo sapiens until roughly 20,000 years ago, may have also posed a challenge to modern humans in terms of power. However, many experts agree that early Homo sapiens were not much different from the burly Neanderthals — the biggest evolutionary change had already taken place roughly 2 million years ago when human ancestors became serious runners.

So in a hypothetical competition, if you wanted to bet on modern Olympic athletes besting earlier humans, choose the endurance events such as the triathalon or soccer. Otherwise the power sports would belong to human ancestors, and for good reason.

The power athletes

For instance, before 2 million years ago, the earliest human ancestors such as Australopithecus afarensis (represented in part by the famed "Lucy" fossil) had just come down from the trees in an evolutionary sense. Their bodies still reflected chimp features such as longer arms and a stronger upper body built for fighting and swinging through the trees.

"A big male chimp weighs about 50 kilos [110 pounds], yet could easily rip the arm off someone," Lieberman noted. "You would never want to arm wrestle a chimpanzee."

The build of Australopithecus unsurprisingly continued the ape trend toward male-male physical competition, said David Carrier, a biologist at the University of Utah. The smaller human ancestor would have had an advantage in fighting sports such as wrestling, especially if the size advantage of modern humans was removed.

The same physical advantages would have extended to other Olympic sports as well.

"The Australopiths would really excel at gymnastics and diving because of the greater upper body strength, longer arms," Carrier told LiveScience. "Their short stature and low body mass would also have greatly increased their ability to do flips and spins because of the low rotational inertia of their body."

Yet when it came to running, Australopithecus found itself in an awkward position of having just learned to walk comfortably on two legs.

"Australopithecus represented a biped on the ground with much more climbing ability and without striding locomotion," said Ian Tattersall, anthropologist and curator for the American Museum of Natural History in New York City. "Running would have been possible, but probably not as efficient."

The most dangerous game

The ability to run for long distances changed human athleticism — as well as history — and came around the 2-million year mark, allowing human ancestors such as Homo erectus to hunt seriously for the first time.

Early hunters only had sharpened wooden sticks and clubs, which meant that success in catching and killing prey relied on the difference between human marathon running and animal sprinting.

"Most human sports that we value the most (with exception of power sports) involve this incredible ability to run or do aerobic capacity," Lieberman said. "That's really rare. Very few animals adapted for endurance."

Lions can run about twice as fast as the fastest Olympic sprinters over short distances to catch their prey. Early humans relied instead upon tiring their prey by running them down to exhaustion, combining a springy step with sweat glands all over the body that prevented overheating.

Modern Olympic marathoners could take full advantage of their running to beat early human ancestors such as Australopithecus in a long-distance race. Even contemporary people who still rely on persistence hunting without long-range weapons can run with equal ease, such as the Tarahumara of northern Mexico.

"Kids on lunch break will go run ten miles," Lieberman noted. "The Tarahumara used to run deer down to exhaustion."

A caveman can do it

The long-running Homo sapiens may seem very different from its relative the Neanderthal, which overlapped with prehistoric humans on Earth until it vanished about 20,000 years ago. But experts say that the similarity is greater than previously thought, and that our modern perceptions have become skewed by modern living.

"If you compare [Neanderthals] to yourself or most people probably living in mechanized urban areas, the Neanderthals would appear very strong," said Erik Trinkaus, a physical anthropologist at Washington University in St. Louis. "However, if you compared them to early modern humans, the Neanderthals and early humans would not appear very different."

Trinkaus said that prehistoric humans would have developed similar or greater strength due to their hunter-gatherer lifestyle, a view that he has gradually come to embrace during his professional career.

"Going to the gym, going on a bike ride, even most Olympic training doesn't do the same thing as having the same serious level of lifting, walking that people have done in the past," Trinkaus explained.

Some debate still exists as to whether the main physiological differences made a difference, such as wider-spaced hips making Neanderthals less efficient at long-distance running. But for the most part, other experts agree that even some humans today likely resemble Neanderthals in physical terms.

"Neanderthals were somewhat shorter and stockier than the average sapiens, but there are modern humans with the same proportions," Carrier said. Lieberman described Neanderthals as "basically like robust early humans."

Both Homo sapiens and Neanderthals certainly shared a definitive advantage over the earlier Australopithecus — males and females became much more equal in body strength and size. Modern Olympic athletes have increasingly embraced that trend."

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Humans, Time of Origin

The lineages of humans and chimpanzees, our closest relatives, diverged from one another about 4.1 million years ago, according to a new estimate that is said to be far more precise than previous ranges for this critical evolutionary moment.

However, the claim is a bad match with previous estimates based on fossil evidence and other genetic work.

Asger Hobolth of North Carolina State University and his colleagues arrived at the new estimate of " the time we became human," or the time in the past when descendents of the human-chimp ancestor split into human and chimp, by statistically comparing DNA from four regions of the human, chimp and gorilla genomes.

The new divergence date is considered fairly recent, maybe too much so. Previous estimates, based on fossil evidence, put the most recent common ancestor of humans and chimps on Earth anywhere from 2 million to 10 million years ago, Hobolth said.

"Primate evolution is a central topic in biology and much information can be obtained from DNA sequence data," Hobolth said in a prepared statement.

The team also came up with a fairly large estimate for the size of the ancestral population of the primates just before human and chimp species evolved from it—about 65,000 individuals. Other primates would have existed at the time, too, but not all were ancestors of ours.

The divergence date has been a matter of hot debate at least since the publication in 1859 of Charles Darwin's "The Origin of Species."

The new estimate supports claims that recently discovered primate fossils, the Millennium man (Orrorin tugenesis) and Sahelanthropus, are not on the human lineage but belong rather to an ancestral lineage from which both humans and chimps evolved.

The results are detailed in the February issue of the journal PLoS Genetics.

The new estimate fails to square up with previous molecular estimates for the divergence date, not to mention the fossil evidence on hominids, said Ian Tattersall, a paleoanthropologist at the American Museum of Natural History.

"You have bipedal hominids by 4 million years ago, and 4.2 million years ago you have definite fossil evidence of bipeds around the place, part of a knee joint and a lower ankle joint that are pretty good indicators," Tattersall told LiveScience. "And then you have more arguable fragmentary back 6 million years ago."

"Chimps are knuckle-walkers and hominids are bipeds," he said, "and it's inconceivable that you could have a common ancestor to both at 4 million years ago when you already have evidence in the hominid lineage that there were bipeds already around at that time."

Timeline of Human Evolution

The timeline of human evolution is long and controversial, with significant gaps. Experts do not agree on many of the start and end points of various species. So this chart involves significant estimates.

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Oldest Upright Walker

A 6 million-year-old early relative of modern humans apparently walked on two feet, pushing back the origins of so-called bipedalism, according to a new study of a fossil found in Kenya.

"I would say at this point it’s the earliest fossil hominin that we can clearly identify as bipedal," said paleoanthropologist William Jungers of Stony Brook University, who conducted a quantitative analysis with Brian Richmond of George Washington University of a fossilized femur bone from the species named Orrorin tugenensis. It is one of the earliest known pre-humans.

The researchers compared the shape of this thigh bone to those of modern humans, apes and other early hominins, including Australopithecus (the species to which the famous "Lucy" fossil belongs). The team determined that the femur bears the signatures of bipedalism, or walking upright on two feet.

The research, funded by the National Science Foundation and Stony Brook and George Washington universities, is detailed in the March 21 issue of the journal Science.

Carol Ward, an anatomist at the University of Missouri-Columbia who was not involved in the research, said the team's findings are significant.

"No detailed study had ever been done on this fossil, and they did a very solid comparative metric analysis," she said.

Debate about tree-climbing

What's special about O. tugenensis, and other early humans that lived between 6 million and 2 million years ago, is that they not only travelled on the ground on two legs but also retained the ability to climb trees, Jungers said.

"These are bipedal walkers that were also using the tress for food, sleeping and escaping from predators," Jungers told LiveScience. The researchers think O. tugenensis was a climber because of a finger bone also found belonging to the species. The finger is curved, Jungers said, a sign that it was used to grasp trees.

Ward said she isn't convinced this species or its later relatives spent a lot of time in trees.

"Everyone agrees they were well-adapted to walking upright on the ground," she said. "People differ on how important tree climbing was. I think we can't say yet. We need more fossils."

Eventually, the ancestors of modern humans completely lost their expert climbing abilities.

"What happens at about 2 million years ago is really fascinating, because you relinquish this very successful body plan, and what emerges is a body plan that's much more similar to yours and mine," Jungers said.

At this point, our ancestors gave up their curved finger bones and gained longer hind legs, perfect for walking long distances and running but not as well suited to scrambling around trees.

The O. tugenensis fossils were discovered in 2000 by a team led by French researchers Martin Pickford and Brigitte Senut. The find was dubbed "Millennium Man."

Pickford and Senut were the first to propose that the species was bipedal, but it wasn't until Jungers' and Richmond's new study that this could be confirmed.

Clarifying the hominin ancestry

The fossils' discoverers had suggested that Orrorin was a direct ancestor of modern humans, with special similarities to us. Jungers and Richmond found that these ancient fossils actually have much more in common with Australopithecus, an extinct early hominin made famous by the discovery of "Lucy." Australopithecus appeared about 4 million years ago, 2 million years after O. tugenensis.

Ward agreed that the new study disproved the hypothesis that Orrorin was a direct modern human ancestor.

"This certainly puts the nail in the coffin on that idea," she said. "They’ve very carefully demonstrated that it looks like Australopithecus."

Both Australopithecus and O. tugenensis were smaller than modern humans and stocky, Jungers said. They had big teeth, projecting faces and small brains, closer to the size of chimpanzee brains than ours.

Though O. tugenensis was not our direct ancestor, it was part of the group of early hominins that eventually gave rise to our genus Homo, as opposed to the related group from which chimpanzees emerged. The study of the Orrorin fossils helps scientists narrow down when humans and chimpanzees split.

"This clearly post-dates that split, so it gives us a minimum date of six million years ago for humans splitting off," Jungers said."

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Early Humans Were Poor Climbers

Our ancient human ancestors traded in the ability to climb trees for the power to walk on two legs, but it is unclear when this happened in evolutionary time.

A new study could help pin down the timing of this exchange, revealing that human ancestors as far back as 4 million years ago didn't have quite the climbing skills of modern chimpanzees, so climbing was phasing out by this time.

The evidence: Early humans lacked the ankle structure that assists chimps in climbing, according to anthropologist Jeremy DeSilva of Worcester State College in Massachusetts.

Wild comparison

DeSilva videotaped wild chimpanzees — our closest living animal relatives — in Uganda to study their bodies while climbing. He measured the angle of dorsiflexion, or how far the ankle could rotate so that the toes point upward, and found that chimps can make much more extreme ankle rotations than modern humans.

To investigate whether early hominins were more like modern humans or chimpanzees, DeSilva analyzed the ankle bones in fossils of human ancestors at various times from 1.5 million to 4 million years ago. He discovered that early humans during this span have dorsiflexion ranges similar to those of modern humans, and couldn't have climbed trees in quite the same way as chimps do, if they climbed at all.

"Frankly, I thought I was going to find that early humans would be quite capable, but their ankle morphology was decidedly maladaptive for the kind of climbing I was seeing in chimps," DeSilva told LiveScience. "It kind of reinvented in my mind what they were doing and how they could have survived in an African savannah without the ability to go up in the trees."

Since tree climbing is useful both for foraging food and for hiding from predators, the benefits of walking upright must have been great to make humans give up their ankles more suited for climbing.

Other research suggests that early humans at this time had limb proportions similar to other mammal species that are particularly aggressive. Perhaps early humans used aggression to discourage predators from targeting them.

Plus, walking on two feet not only enabled travelling long distances, but also escaping more quickly on the ground from predators.

Energy required

Bipedalism, or the ability to walk upright, is thought to demand sacrificing climbing skills because the body proportions required for both are different.

Specifically, walking on two legs requires a lot of energy to lift the foot and ankle, so minimizing their weight is important. But climbing requires bones in different places on the foot than walking does, and keeping both sets of bones would be too heavy, DeSilva said.

"I think by 3 [million] to 4 million years ago that tradeoff was occurring," he said. "Our ancestors were becoming very capable upright walkers, and it came at the expense to our ability to climb trees."

Will Harcourt-Smith, an anthropologist at the American Museum of Natural History in New York, said the DeSilva study was unique in comparing ancient fossils to observations of living animals' movements.

"More studies doing that will help us," he said. "This is a very nice finding, but I think it's only part of the story. And it doesn’t preclude these fossil creatures from being able to climb at all. But if they did climb, they were climbing in different ways than chimps do."

Harcourt-Smith said DeSilva's ankle data could be compared to measurements of other body parts from other studies to form more of a complete picture.

More to learn

DeSilva said other research found that many early humans' toes were not likely able to grasp as well as chimpanzees' do, which would be a useful attribute for climbing. And measurements of early human knees and hips also indicates that these bones weren't well adapted for climbing.

However, fossils of upper body parts, such as the strong arms and curved fingers on some early human specimens, suggest they retained some of their climbing skills.

Ultimately, to get to the bottom of the issue, more fossil discoveries are essential.

"The thing we really need is to find more fossil hominins between 5 and 7 million years old," Harcourt-Smith said. "We need to find more creatures that would have been closely related from roughly the time of this last common ancestor between chimps and humans."

The study is detailed in this week's issue of the journal Proceedings of the National Academy of Sciences."

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Anthropologists Examine Relatives and Friends

Brian Hare's Photograph

Photo taken by Vanessa Woods
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To find out what makes us human, Brian Hare asks our closest relatives and best friends.

As a new and newsworthy assistant professor of biological anthropology and anatomy at Duke, he’s examining the social abilities of chimpanzees and bonobos, the two endangered species of ape with which we share about 99 percent of our genes.

This summer, the focus is on “xenophobia” (ZEE-no-phobia), the fear of strangers."

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Dr. Brian Hare, Assistant Professor of Evolutionary Anthropology, Duke University

Brian is only 32, but has already had a stellar career as a researcher into the social skills of chimpanzees and their close relatives, bonobos. Alan, having entertained the journalists, was making friends through the thick plate glass with Hondo, the alpha male of the NCZoo chimp group when Brian arrived. When the two men sat down to chat, Hondo, who had been calmly munching breakfast, suddenly hurled his whole body at the glass where Alan was sitting.

And along the way we’ll be taking a look into the heads not only of our fellow primates but also of our own species – as well as a species that’s much more distantly related – dogs – who are apparently better able to understand certain human social cues than our primate cousins. Brian Hare was actually the first to suggest this, way back when he was an undergraduate.

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