The next time you visit a zoo, make a point of walking by the ape cages. Imagine that the apes had lost most of their hair, and imagine a cage nearby holding some unfortunate people who had no clothes and couldn’t speak but were otherwise normal. Now try guessing how similar those apes are to us in their genes. For instance, would you guess that a chimpanzee shares 10 percent, 50 percent, or 99 percent of its genetic program with humans?
JARED DIAMOND, THE THIRD CHIMPANZEE
THE APE’S REFLECTION?
The subject of human evolution along with the rest of the animal kingdom has always been contentious and emotional. For religious reasons, even today a significant minority of Americans reject the idea that humans are related to the rest of the animal kingdom or that they are just another animal species—even though this fact is not controversial in almost any other modern developed nation in the world. Yet some polls show that a high percentage of Americans accept the idea of evolution in plants and other animals—just not humans.
Ironically, so much research and interest have been focused on the evolution of humans that it is one of the best-supported examples of evolution of all. An entire branch of anthropology (physical anthropology and human paleontology) is devoted to the fossil record of our nearest relatives. Thousands of scientists worldwide are working on an array of research topics in this field—far more than study dinosaurs or any other prehistoric creatures. Literally hundreds of thousands of specimens of fossil hominins (members of the human subfamily, or the subfamily Homininae) are stored in museums all over the world. The number of specimens is so overwhelming, and the wealth of detail about human evolution is so impressive, that if we were talking about any other species on the planet, it would be a slam-dunk case of evolution, as well documented as that of any family of animals. But so many people hold nonscientific objections to the idea that it receives unfair scrutiny, is distorted, and is denied outright. If the same volume of overwhelming evidence were brought to bear on any other issue, there would be no controversy at all.
But even if we did not have the incredible fossil record of humans, the evidence is still overwhelming. All we have to do is look in a mirror. As early as 1735, the founder of modern classification, Carolus Linnaeus, gave humans the scientific name Homo sapiens (thinking man) and diagnosed our species with the Greek phrase “Know thyself.” In 1766, Georges-Louis Leclerc, Comte de Buffon, wrote in volume 14 of Histoire naturelle that an ape “is only an animal, but a very singular animal, which a man cannot view without returning to himself.” Other French naturalists like Georges Cuvier and Étienne Geoffroy Saint-Hilaire commented on the extreme anatomical similarity of apes and humans, although they refused to actually say that humans are a kind of ape. The pioneering French biologist Jean-Baptiste Lamarck explicitly argued in Philosophie zoologique in 1809:
Certainly, if some race of apes, especially the most perfect among them, lost, by necessity of circumstances, or some other cause, the habit of climbing trees and grasping branches with the feet,…, and if the individuals of that race, over generations, were forced to use their feet only for walking and ceased to use their hands as feet, doubtless…these apes would be transformed into two-handed beings and…their feet would no longer serve any purpose other than to walk.
The issue clearly was a critical one when Charles Darwin published
On the Origin of Species in 1859. His book was already controversial, so he tried his best to downplay the issue of human evolution. In the entire book, he wrote only one phrase: “Light will be shed on the origin of man, and his history.” Although Darwin was reluctant to say more at that time, his supporter Thomas Henry Huxley jumped into the breach and in 1863 published
Evidence as to Man’
s Place in Nature. In this book, Huxley described and illustrated the detailed anatomical similarity of every bone and muscle and organ in the great apes and in humans (
figure 24.1). Finally, in 1871, Darwin published his own thoughts in
The Descent of Man, although he focused mostly on topics such as sexual selection, without even mentioning fossils. At that time, there was still no fossil evidence for human evolution (other than Neanderthals, who had been misinterpreted).
Figure 24.1
Benjamin Waterhouse Hawkins’s illustration of the extreme bone-by-bone similarity of the skeletons of apes and humans. (From Thomas Henry Huxley, Evidence as to Man’s Place in Nature [London: Williams & Norgate, 1863])
Jump forward in time 90 years. Unbeknownst to Darwin or any other biologist before the 1960s, another source of data clearly shows our relationships to apes and the rest of the animal kingdom: DNA. Some of the very first molecular techniques demonstrated that human DNA and chimp and gorilla DNA are extremely similar. When the serum of antibodies of humans and of apes is put in the same solution, the immune reactions are much stronger than those with humans and any other animal, suggesting that the immunity genes of humans and of apes are most similar.
Then in the late 1960s, a technique called DNA-DNA hybridization was developed. A solution of DNA of an ape and a human is heated until the two strands of the double helix unzip. Then the mixture is cooled, and each strand binds to the nearest strand, creating some DNA with one strand from the human and the other from the ape. (Some of the strands of the ape’s DNA bind to other ape strands, and some of the strands of the human’s DNA bind to other human strands, but of greatest interest are the double helices of hybrid DNA.) When the solution with the hybrid DNA is reheated, the more tightly bonded the hybrid strands (which reflects how similar they are), the higher the temperature needed to unzip them. Doing this with the DNA of chimps, gorillas, other apes, plus monkeys, lemurs, and nonprimate animals gives a rough measure of how similar each is to humans—and, once again, chimp DNA is virtually identical to human DNA.
Then, in the past 20 years, technological leaps like the polymerase chain reaction (PCR) have made it possible to directly sequence the DNA not only of humans, but of many other animals and plants. The entire genome of humans was sequenced in 2001, and that of chimps in 2005. When they were compared, the result was exactly the same as that obtained from DNA-DNA hybridization: humans and chimps share 98 to 99 percent of their DNA. Less than 1 to 2 percent of our DNA differentiates us from chimps and from gorillas as well. This is because about 60 to 80 percent of our DNA is “junk” that is never read or used, but is carried around passively generation after generation. Some of this junk is endogenous retroviruses (ERVs), which are remnants of viral DNA inserted into our genes when some distant ancestor was infected, and still carried around even though it no longer codes for anything. A smaller percentage is structural genes that code for every protein and structure in our body, including genes we no longer use. The 1 to 2 percent that distinguishes us from chimps are regulatory genes, the “on–off switches” that tell the rest of the genome when to be expressed and when not to be. They are the reason that humans look so different from other apes, even though our genes are nearly identical.
For example, all apes and humans have the structural genes for a long tail, but do not express those genes, except in rare cases where the regulatory genes fail. When such an error occurs, humans grow a long bony tail. Birds also have the genes for a long bony, dinosaurian tail, inherited from their raptor ancestors, not the stubby “parson’s nose” fused tailbones found in modern birds. Once in a while, the regulatory genes fail and birds hatch with dinosaur tails. Likewise, living birds have toothless beaks, and no longer the teeth of their dinosaur ancestors (
chapter 18), but they still have the genes to make teeth. Experimentally grafting the mouth epithelial tissue of a mouse into a chick embryo produced a bird with teeth. But the teeth that grew were not mouse teeth, but dinosaur teeth! Thus all animals have many ancient genes in their DNA that are no longer expressed, but it takes only some sort of modification of gene regulation to resurrect primitive features.
Figure 24.2
Molecular phylogeny of apes and humans, showing their genetic distance from one another based on mitochondrial DNA. All human “races” are much more similar to one another than two populations of gorillas or chimpanzees are to each other. (Modified from Pascal Gagneux et al., “Mitochondrial Sequences Show Diverse Evolutionary Histories of African Hominoids,” Proceedings of the National Academy of Sciences USA 93 [1999], fig. 1B; © 1999, National Academy of Sciences USA)
The extreme similarity of the genes of humans to those of the two species of chimpanzee (common chimp [
Pan troglodytes] and pygmy chimp, or bonobo [
P. paniscus]) should, all by itself, be overwhelming and convincing proof of our close relationship. Despite some people’s gut reactions and religious ideas, humans are indeed the ape’s reflection. Biologist Jared Diamond puts it this way: imagine that some alien biologists came to Earth, and the only biological samples they could obtain were DNA. They sequenced many different animals, including humans and the two chimps. Based on these data alone, they would conclude that humans are just a third species of chimpanzee. Our DNA is more similar to that of the two species of chimp than the DNA of any two species of frog are similar to each other, and even more similar than the DNA of lions and tigers are to each other. Indeed, the differences among the DNA of all the human “races” are smaller than are the differences between the DNA of different populations of chimpanzees from various regions of Africa (
figure 24.2)! This suggests two things. First, the genetic differences among the human “races” are tiny and trivial, and are much less significant than many people realize. And second, the big differences between the appearances of chimps and humans are caused by tiny changes in the regulatory genes, which have huge results.
Case closed: humans are slightly modified apes. The evidence from genes, as well as from anatomy, is overwhelming. The DNA in every cell in your body is a testament and witness to your close relationship to chimps, no matter how much this fact makes some people uncomfortable or upset. We know this without a single fossil human showing the transition from apes. But how long ago did humans and apes diverge?
CLOCKS IN ROCKS
Scientists have approached the question of when the ape and human lineages split from each other in two ways. One is to search for fossils that are progressively more ape-like than human-like. This strategy is being tried all the time as exploration continues, although its success depends on the luck of finding the right rocks of the right age and hoping that a primitive hominin fossil might be preserved in them. Human bones tend to be very rarely fossilized, so even in beds with humans fossils, there may be only a few scraps of hominin teeth or jaws compared with the hundreds of specimens of other mammals, such as pigs or antelopes or mastodonts. Nonetheless, as we shall see in
chapter 25, paleoanthropologists have spent decades in the field trying to find these elusive hominin fossils, since an important discovery will make a career and burnish a reputation.
Once hominin fossils are found, the next trick is to obtain a reliable date for them. Many hominin fossils are discovered in caves or other places where there is no material that can give a useful date. If the specimen is younger than about 60,000 years (latest Ice Age to Holocene in age), the organic material in the fossil can be dated directly using carbon-14 dating (or radiocarbon dating). This technique is widely employed by archeologists to date human artifacts (most of which are younger than 60,000 years old) and by paleontologists to date late Ice Age fossils. For example, the fossils found in the La Brea Tar Pits in Los Angeles are no older than about 37,000 years, so they have been dated repeatedly using the radiocarbon technique.
For older fossils, however, dating is much more complicated. Radiocarbon dating no longer works on material older than 60,000 years (although the best labs today can sometimes push it out to 80,000 years). The best method to use on older fossils is potassium-argon (K-Ar) dating (or its newer version, argon-argon [40Ar/39Ar] dating). With this technique, a fossil cannot be dated directly, by analyzing material either from the specimen or from the sedimentary layers in which it was found. Instead, what is dated are the crystals that formed when they cooled out of a volcano, either a lava flow or a volcanic ash fall. Once the volcanic crystals cool, they lock the unstable parent isotope, potassium-40, into their lattices. As the crystals age, the unstable potassium atoms spontaneously decay, or break down, to form a daughter isotope, argon-40. The rate of decay is very well known, so by measuring the ratio of parent atoms to daughter atoms, geologists can calculate the age of the crystals.
As with any other technique in science, there are limitations and pitfalls that have to be avoided. Because dating is a measure of the time since a crystal cooled and locked in the radioactive parent atoms, potassium-argon dating works only with rocks that cool down from a molten state, or igneous rocks (such as granites or volcanic rocks). A good geologist will tell you that the crystals in a sandstone or any other sedimentary rock cannot be directly dated. Those crystals were recycled from older rocks and have no bearing on the age of the sediment. But geologists long ago circumvented this problem by finding hundreds of places all over Earth where datable volcanic lava flows or ash falls are interbedded with fossiliferous sediment, or where intruding magma bodies cut across the sedimentary rocks and provide a minimum age. From settings such as these, the numerical ages of the geological time scale are derived, and their precision is so well resolved that we know of the age of most events that are millions of years old to the nearest 100,000 years.
If the crystal structure has somehow leaked some of its parent or daughter atoms, or allowed atoms to enter the lattice and contaminate the crystal, the parent/daughter ratio is disturbed and the date is meaningless. But geologists are always on the lookout for this problem, running dozens of samples to determine whether the age is reliable and cross-checking their dates against other sources of determining age. The newest techniques and machinery are so precise that a skilled geologist can spot an error in almost any date and quickly reject dates that don’t meet very high standards.
By these methods, most of the fossils found in Africa have been dated very precisely, establishing their ages over the past 5 million years (
chapter 25). Anthropologists have frequently collaborated with geochronologists to find fresh ash layers with many unweathered crystals of the appropriate minerals (typically potassium feldspars, but also micas like muscovite and biotite). There have been a few missteps along the way, but generally the age framework of most hominin fossils is well established. In addition, if volcanic ash layers are not present in a given area, then paleontologists can use the differences in fossil assemblages through time to obtain a rough sense of the age of a locality, since the same fossil assemblage occur elsewhere associated with a volcanic ash date.
But what about the fossil record? The story starts with important fossils that were found in the Siwalik Hills of Pakistan. This amazing sequence of rocks spans much of the Oligocene, Miocene, and Pliocene epochs of geologic history and is incredibly fossiliferous. These deposits represent the flood of river sediments that were shed across South Asia as the Himalayas slowly rose high in the sky and that eroded to form the Siwaliks. They have been studied by paleontologists and geologists since 1902, when British geologist Guy Pilgrim did pioneering research throughout South Asia, which was a British colony.
Over the past century, the Siwaliks have yielded huge collections of fossil mammals that offer a very detailed picture of evolution in South Asia during the Miocene. Thanks to the tense nature of Indian and Pakistani politics and American policy toward both countries, Pakistan owed the United States millions of dollars for all the military hardware it had bought. As a result, from the 1970s through the 1990s, there was a lot of grant money (especially from the Fulbright Foundation) for American scholars to go to Pakistan and undertake important research. Lots of paleontologists jumped on the Fulbright opportunity, and there was a flood of studies on the fossils and geology of the Siwalik Hills and nearby areas. Thanks to an abundance of volcanic ash and a technique called paleomagnetic stratigraphy, the Siwalik fossils are extremely well dated. Today, of course, the political situation is so dangerous that few Americans can travel there, and even researchers from other countries who have no ties to the United States are threatened by the pro–Al Qaeda and pro-Taliban tribes in many regions.
But in 1932, paleontologist G. Edward Lewis of the Smithsonian Institution was working in the Tinau River valley in the Nepalese Siwaliks and recovered a jaw that looked very much like that of a primitive hominin. It had relatively small canines, and its shape was more like a broad semicircle in top view (typical of human jaws) than like the
U-shaped jaw of apes, with its huge canines on a flat lower chin and long parallel back parts. In the 1960s, anthropologist David Pilbeam of Harvard and primatologist Elwyn Simons of Yale and then Duke and others began to champion the view that this jaw (named
Ramapithecus by Lewis) was the oldest known hominin fossil. (Rama is one of the Hindu gods, and
pithecus is Greek for “ape”; there are also primates named after the Hindu gods Shiva and Brahma.) Since some of the specimens dated back to 14 million years ago in the well-calibrated Siwalik sequence, this placed the split between apes and hominins at least 14 million years ago. Through the 1960s and 1970s, every student of anthropology, primate evolution, and human paleontology learned that
Ramapithecus was the “first hominin.”
CLOCKS IN MOLECULES
There is an approach other than radiocarbon and potassium-argon techniques to dating the time of divergence between two groups of animals: the molecular clock. As early as 1962, the legendary molecular biologists Linus Pauling (winner of two Nobel Prizes) and Emile Zuckerkandl were among the first to use molecular methods to draw a tree of evolutionary relationships among organisms, the first evidence of evolution to emerge from our own cells and DNA. Pauling and Zuckerkandl noticed not only that the number of amino-acid differences in hemoglobin molecules matched the branching sequence of the animals in their study, but that the number of changes was proportional to how long ago these creatures had diverged from one another over time. A year later, another pioneer in molecular biology, Emanuel Margoliash, noted:
It appears that the number of residue differences between cytochrome c of any two species is mostly conditioned by the time elapsed since the lines of evolution leading to these two species originally diverged. If this is correct, the cytochrome c of all mammals should be equally different from the cytochrome c of all birds. Since fish diverges from the main stem of vertebrate evolution earlier than either birds or mammals, the cytochrome c of both mammals and birds should be equally different from the cytochrome c of fish. Similarly, all vertebrate cytochrome c should be equally different from the yeast protein.
All these data suggested that molecular changes have accumulated through time as different groups of animals branched apart, and that the rate of change of molecules is proportional to the time the lineages split or diverged.
Meanwhile, the evidence that most of the DNA of any animal is “junk” or at least nonfunctional began to emerge. So much of the genome is simply never read when the genes are expressed and thus is invisible to natural selection, or adaptively neutral. Pioneering work by Japanese biochemist Motoo Kimura, in particular, established that most of the molecules in DNA are unaffected by what happens to the organism. These adaptively invisible molecules can spontaneously mutate, and there is no selection to weed them out or favor one version over another. Over time, these mutations continue to accumulate at a regular rate, ticking like a clock. As long as natural selection cannot “see” these changes, the ticking of the “molecular clock” is a good method of estimating divergence time in the geologic past between any two lineages. The only thing needed is calibration by using well-established divergence times of key evolutionary splits, as established in the fossil record.
Soon many molecular biologists were working hard on molecular clock estimates of the branching history and timing of divergence of many groups of animals. Again and again, work by the late Vincent Sarich and Allan Wilson at Berkeley showed that the molecular clock estimate for the divergence between humans and chimps is only 7 to 5 million years ago and no earlier than 8 million years ago, not the 14 million years ago that Ramapithecus suggested. Yet the paleontologists stuck by their guns. They distrusted the molecular clock method as unproven and unreliable because it did indeed give some very strange and ridiculous results every once in a while. (This still happens, and we do not always know why.)
As the controversy got more and more heated during the 1970s and 1980s, the major players got into shouting matches at meetings and contentious debates in journals. Sarich and Wilson were convinced that their data were reliable and something must be wrong about
Ramapithecus or its age. Sarich was a burly, towering, impressive figure with a natty beard, a loud voice, and strong opinions who did not mind ruffling feathers and offending people if necessary. In 1971, he said, “One no longer has the option of considering a fossil older than about eight million years as a hominid no matter what it looks like.” This, of course, upset researchers like Simons and Pilbeam, who kept insisting that
Ramapithecus proved that the molecular biologists were wrong.
The impasse was finally broken by another discovery in the Siwaliks. In 1982, Pilbeam reported on newly discovered specimens that included not only a more complete lower jaw of Ramapithecus, but also a partial skull. With the addition of the skull, the specimen now looked much more like a fossil orangutan that had been named Sivapithecus by Guy Pilgrim in 1910 when the Siwaliks were first explored. The lower jaw of Ramapithecus was just the jaw of a fossil relative of the orangutan that happened to look like a hominin. Soon, the anthropologists were forced to retreat and acknowledge their error, which ceded the victory to Sarich and Wilson and molecular biology. Now that paleontologists knew that there were no hominin fossils as old as 14 million years, the questions then became: What is the oldest hominin fossil? And would it indeed fit the prediction from Sarich and Wilson that it is no older than 8 million years old?
“TOUMAI”
Through the past 25 years, paleoanthropologists have been working hard all over the world to push back the fossil record of hominins into older and older beds. As discussed in
chapter 25, humans evolved in Africa, and the oldest fossils are found there. Although the early work focused on South Africa, and then on Kenya and Tanzania, since the 1970s the effort has concentrated on even older beds in places like Ethiopia.
Since the discovery of “Lucy” (
Australopithecus afarensis) in 1974 (
chapter 25), there has been a major discovery of even older specimens every few years. In 1984, fossils were found in Kenya of a poorly known species called
Australopithecus anamensis. This material is much more primitive than “Lucy” and dates to 5.25 million years ago. Then in 1994, an even more primitive species was found in Ethiopia. Named
Ardipithecus ramidus, it was based on a few scrappy fossils until 2009, when Tim White and his co-workers announced a partial skeleton and many more fossils. Now
Ardipithecus consists of a number of limb elements and even a partial skull. Recent discoveries of an even older species,
Ardipithecus kaddaba, push the genus back to 5.6 million years ago.
Meanwhile, a French-British-Kenyan team led by Martin Pickford was working in the Tugen Hills, an area of Kenya that is much older than the classical deposits at Olduvai Gorge and Lake Turkana. In 2000, they announced the discovery of an even older hominin called
Orrorin tugenensis. Much better fossils were reported in 2007.
Orrorin is known from only about 20 specimens (the back of the jaw, the front of the jaw, isolated teeth, fragments of the upper arm bone and thighbone, and finger bones). The teeth (as far as they are known) are very ape-like, but the hip region of the thighbone clearly shows that
Orrorin was bipedal. Like other Kenyan deposits, the Tugen Hills contains dated volcanic ashes, which place the age of the
Orrorin fossils at between 6.1 and 5.7 million years old.
Thus the hominin fossil record now extends back to at least 6 million years ago, within the window predicted by molecular clocks at about 7 to 5 million years for the split between hominins and apes. But where would one find slightly older beds that might preserve fossil hominins? By 1995, French paleontologist Michel Brunet had spent many years working on Miocene mammals around the world. He specialized in working on some of the most dangerous and remote fossil sites. Brunet had been strafed by fighter jets in Afghanistan, arrested in Iraq, lost a collaborator to malaria in Cameroon, and been held at gunpoint in Chad. By the mid-1990s, he had been digging in the Miocene beds of Chad (once a French colony) for many years.
The conditions in the Djurab Desert in Chad are not exactly easy to tolerate. Brunet was approaching 60, and working in the desert would have been a challenge for a much younger man. Even though the temperatures can reach 43 to 49°C (110 to 120°F), Brunet had to wrap his head in cloth and wear a ski mask and goggles to protect himself from the sand that blows into eyes, ears, nose, and mouth. The temperatures in the shade can be so hot that water bottles can spontaneously explode. As Brunet and his colleagues swept the desert floor looking for fragments of bones and teeth, they had to be careful not only because of the killer heat and the howling winds and sand, but also because of the buried land mines left by combatants in one of the many tribal wars. On January 23, 1995, he found a jawbone of a primitive hominin that was 3.5 million years old, the first such find outside South or East Africa. It was later named Australopithecus bahrelghazali.
The following July, he met in Addis Ababa with Tim White at the National Museum of Ethiopia to compare the hominin fossils he had found in Chad with those that White had unearthed in Ethiopia. Brunet told White that he knew of an older formation below the one that had yielded the jaw he brought to show him. The older formation contained the fossils of extinct gerbils and other mammals that placed it between 7 and 6 million years in age. White was doubtful because gerbils indicate dry climates, and he thought that hominins would not be found there. While they were in the museum, Brunet bet White that he would find older hominins, since he was working in older sediments. “I will win,” he said.
Fast-forward to 2001. For six more years, Brunet worked on the older beds, which are late Miocene in age and contain fossils that suggest they are 7 to 6 million years old. Brunet and his colleagues formed the Mission Paléoanthropologique Franco-Tchadienne (MPFT), a collaboration between the University of Poitiers and the University of N’Djamena in Chad. Brunet and three Chadian crew members were working in the broiling heat at a locality called Toros-Menalla. Suddenly, Ahounta Djimdoumalbaye bent down and looked closer at an object protruding from the ground. He called to Brunet and the rest of the crew, and they soon saw that Djimdoumalbaye had found a very important specimen. It looked somewhat like an ape skull, but it also had hominin features (
figure 24.3). They quickly recovered it, saturated it with hardeners, and carried it back to camp.
Even though Brunet had not finished analyzing the specimen since bringing it to the University of Poitiers, the rumor mills were buzzing. Everyone was speculating about what had been found, based on a few leaks from people who saw pictures of or heard about the skull. Brunet had no choice but to publish a preliminary analysis before false information was spread. On July 11, 2002, his article appeared as the leading paper in the world’s preeminent scientific journal, Nature. Brunet named the specimen Sahelanthropus tchadensis, after the Sahel region of Chad, where it had been found, and the French spelling “Tchad,” But he and his collaborators nicknamed it “Toumai,” which means “hope of life” in the Dazanga language of Chad.
Sahelanthropus consists of only a skull, with no jaw or any other part of the skeleton. It was also badly crushed and sheared diagonally, so it looks very odd and asymmetric in its original form. Technicians and computer experts have used morphing software to retrodeform the skull and show its true shape before it was smashed and buried. The fossil is about the size of chimp skull, so
Sahelanthropus would have been chimp-size in life. The skull encloses a brain cavity of about 320to 380 cubic centimeters (cc) in volume (compared with modern humans, with over 1350 cc in brain volume). It still has big brow ridges, like apes and many primitive hominins. There are a number of other ape-like features as well, including the relatively primitive cheek teeth.
Figure 24.3
“Toumai,” the skull of Sahelanthropus tchadensis. (From Michel Brunet et al., “A New Hominid from the Upper Miocene of Chad, Central Africa,” Nature, July 11, 2002; courtesy Nature Publishing Group)
Yet as Brunet and his colleagues pointed out,
Sahelanthropus has some features that definitely put it closer to hominins than to chimps or other apes. Its flat face has almost no snout, unlike the face of any ape. It has small canines, unlike the big fangs of apes (even though it appears to be the skull of a male, and most male apes have large canines), and thus its teeth are arranged around the palate in a
C shape, rather than the elongate
U shape characteristic of most apes. Most important, the position of the hole in the bottom of the skull (foramen magnum), through which the spinal cord connects to the brain, is directly below the base of the skull, not tilted to the back of the braincase. This indicates that the skull sat upright over the spinal column, rather than hanging forward from the spine, as in chimps and other apes.
This last point is crucial. As we shall see in
chapter 25, the biases of anthropologists for most of the twentieth century was that brain size was the most important factor influencing human evolution and that features like bipedal erect posture came later. Yet most of the hominins whose fossils have been found in the past 30 years, from “Lucy” to
Ardipithecus to
Ororrin, were clearly fully bipedal, but had small brains. Now
Sahelanthropus, the oldest hominin fossil yet discovered, also shows evidence that its skull sat directly above its spine. Bipedalism is one of the first adaptations that occurred in human evolution, long before our brains got big.
This realization—combined with the flat face, small canines, and hominin-like upper jaw shape—put Sahelanthropus closer to humans than to any ape. Although there are always new discoveries, for now “Toumai” holds the record as the oldest member of the hominin family. And its age, at 7 to 6 million years, is exactly where molecular biologists have been predicting the timing of the chimp–human split for the past 40 years.
SEE IT FOR YOURSELF!
The original fossils of Sahelanthropus, Ororrin, Ardipithecus, Australopithecus, and other earliest hominins are kept in special protected storage in the museums of the countries from which they came (mainly, Ethiopia, Kenya, Tanzania, and Chad). Only qualified researchers are allowed to view these collections or to touch these rarest of treasures.
Many museums have exhibition halls devoted to human evolution, featuring high-quality replicas of the most important fossils. In the United States, they include the American Museum of Natural History, New York; Field Museum of Natural History, Chicago; National Museum of Natural History, Smithsonian Institution, Washington, D.C.; Natural History Museum of Los Angeles County, Los Angeles; San Diego Museum of Man; and Yale Peabody Museum of Natural History, Yale University, New Haven, Connecticut. In Europe, they include the Natural History Museum, London; and Museum of Human Evolution, Burgos, Spain. Farther afield is the Australian Museum, Sydney.
Diamond, Jared M. The Third Chimpanzee: The Evolution and Future of the Human Animal. New York: HarperCollins, 1992.
Gibbons, Ann. The First Human: The Race to Discover Our Earliest Ancestors. New York: Anchor, 2007.
Huxley, Thomas H. Evidence as to Man’s Place in Nature. London: Williams & Norgate, 1863.
Klein, Richard G. The Human Career: Human Cultural and Biological Origins. 3rd ed. Chicago: University of Chicago Press, 2009.
Marks, Jonathan. What It Means to Be 98% Chimpanzee: Apes, People, and Their Genes. Berkeley: University of California Press, 2003.
Sponheimer, Matt, Julia A. Lee-Thorp, Kaye E. Reed, and Peter S. Ungar, eds. Early Hominin Paleoecology. Boulder: University of Colorado Press, 2013.
Tattersall, Ian. The Fossil Trail: How We Know What We Think We Know About Human Evolution. New York: Oxford University Press, 2008.
——. Masters of the Planet: The Search for Our Human Origins. New York: Palgrave Macmillan, 2013.
Wade, Nicholas. Before the Dawn: Recovering the Lost History of Our Ancestors. New York: Penguin, 2007.
