10 natural history September 2018 Where Do We Begin? By Jeremy M. DeSilva G enetic analyses have firmly established that, among extant primates, humans are most closely related to the African apes, and in particular to the chim- panzee and the bonobo. The two species of goril- las are our second cousins. Furthermore, the small differences between human DNA and that of chimpanzees and bonobos reveal that our lineage, that of the hominins, diverged from theirs sometime between 8 million and 6 million years ago. Yet the famous “Lucy” skeleton, a female Australopithecus afarensis discovered in Ethiopia in 1974 by paleoanthropologist Donald C. Johanson of the Cleve- land Museum of Natural History (and later founder of the Institute of Human Origins at Arizona State University), is only 3.2 million years old. That is chronologically around the halfway point from the time of the last common ances- tor. What did our earlier close relatives look like? For years paleoanthropologists simply didn’t know, and so chimpanzees and bonobos became a placeholder for Lucy’s predecessors. But in the mid-1990s, a team led by University of California, Berkeley, paleoanthropologist Tim D. White began to pull back the curtain on this hidden time period, with the announcement of a small collection of 4.4-million-year-old fossils from Ethiopia. Preliminary as- sessment of these fragmentary remains revealed them to be from a new, quite primitive, and ape-like hominin. White first named it Australopithecus ramidus, and then, only a few months later, in a half-page “corrigendum,” Australo- pithecus ramidus became Ardipithecus ramidus (which means “root ground-ape”). Perhaps most tantalizing about this brief announcement was the mention of a partial skele- ton, followed later by White’s quote: “Let’s just say ramidus had a type of locomotion unlike anything living today. If you want to find something that walked like these things, you’d have to go to the bar in Star Wars.” Over a dozen years and three Star Wars prequels would come and go before the world would see the perplexing Ar- dipithecus skeleton. More on that later. As the new millen- nium approached, the paleoanthropological community still knew next to nothing about the dawn of our lineage. All that changed in a twenty-month fossil frenzy that be- gan in the fall of 2000, when Kenyan fossil hunter Kiptam Cheboi discovered jaw fragments in the Tugen Hills of Ke- nya. More fossils were soon recovered, including an upper leg bone, and by February 2001 these roughly 6-million- year-old fossils were analyzed and published by a French team led by paleontologist Martin Pickford at the College of France and anatomist Brigitte Senut at the National Mu- seum of Natural History in Paris. They named this discovery a new species of early hominin, Orrorin tugenensis (mean- ing “original man from Tugen”). Less than five months later, Ethiopian paleoanthropologist Yohannes Haile-Selassie, then at the University of California at Berkeley, now at the Cleveland Museum of Natural History, revealed roughly 5.5-million-year-old fossils from the Middle Awash study area in Ethiopia that represented a second species of early hominin called Ardipithecus kadabba. In July 2001, Ahounta Djimdoumalbaye, then a pale- ontology student at the University of Poitiers, France, was scouring the desolate, but fossil-rich, Djurab Desert in Central Chad—thousands of miles from the Great Rift Val- ley—and discovered a stunningly complete but distorted skull, along with some isolated jaws and teeth. Revealed through publication a year later by a team led by French paleontologist Michel Brunet, these roughly 7-million- year-old fossils were declared the earliest-known hom- inin species, Sahelanthropus tchadensis. In less than two years, our science went from having essentially no fossils from the critical time range to having three new members at the trunk of the human family tree. But what do these fossils reveal about the earliest members of the human lin- eage? Indeed, how do we know they are on our lineage and not the branch leading to chimpanzees, gorillas, or even an extinct side group of apes with no modern descendants? O n the whole, Ardipithecus, Orrorin, and Sahelan- thropus would look and behave much more like an African ape than a human. The Sahelanthro- pus skull has a thick and projecting brow ridge, a gorilla-like face, attachments for strong neck muscles, and a braincase the size of an orange (comparable to that of a chimpanzee). Orrorin and Ardipithecus have curved digits and powerfully built arms, which indicate that our earliest Recently discovered fossils exhibit puzzling combinations of humanlike and ape-like features.
Transcript
10 natural history September 2018
Where Do We Begin?
By Jeremy M. DeSilva
Genetic analyses have firmly established that, among extant primates, humans are most closely related to the African apes, and in particular to the chim-panzee and the bonobo. The two species of goril-las are our second cousins. Furthermore, the small
differences between human DNA and that of chimpanzees and bonobos reveal that our lineage, that of the hominins, diverged from theirs sometime between 8 million and 6 million years ago. Yet the famous “Lucy” skeleton, a female Australopithecus afarensis discovered in Ethiopia in 1974 by paleoanthropologist Donald C. Johanson of the Cleve-land Museum of Natural History (and later founder of the Institute of Human Origins at Arizona State University), is only 3.2 million years old. That is chronologically around the halfway point from the time of the last common ances-tor. What did our earlier close relatives look like?
For years paleoanthropologists simply didn’t know, and so chimpanzees and bonobos became a placeholder for Lucy’s predecessors. But in the mid-1990s, a team led by University of California, Berkeley, paleoanthropologist Tim D. White began to pull back the curtain on this hidden time period, with the announcement of a small collection of 4.4-million-year-old fossils from Ethiopia. Preliminary as-sessment of these fragmentary remains revealed them to be from a new, quite primitive, and ape-like hominin. White first named it Australopithecus ramidus, and then, only a few months later, in a half-page “corrigendum,” Australo-pithecus ramidus became Ardipithecus ramidus (which means “root ground-ape”). Perhaps most tantalizing about this brief announcement was the mention of a partial skele-ton, followed later by White’s quote: “Let’s just say ramidus had a type of locomotion unlike anything living today. If you want to find something that walked like these things, you’d have to go to the bar in Star Wars.”
Over a dozen years and three Star Wars prequels would come and go before the world would see the perplexing Ar-dipithecus skeleton. More on that later. As the new millen-nium approached, the paleoanthropological community still knew next to nothing about the dawn of our lineage.
All that changed in a twenty-month fossil frenzy that be-gan in the fall of 2000, when Kenyan fossil hunter Kiptam
Cheboi discovered jaw fragments in the Tugen Hills of Ke-nya. More fossils were soon recovered, including an upper leg bone, and by February 2001 these roughly 6-million-year-old fossils were analyzed and published by a French team led by paleontologist Martin Pickford at the College of France and anatomist Brigitte Senut at the National Mu-seum of Natural History in Paris. They named this discovery a new species of early hominin, Orrorin tugenensis (mean-ing “original man from Tugen”). Less than five months later, Ethiopian paleoanthropologist Yohannes Haile-Selassie, then at the University of California at Berkeley, now at the Cleveland Museum of Natural History, revealed roughly 5.5-million-year-old fossils from the Middle Awash study area in Ethiopia that represented a second species of early hominin called Ardipithecus kadabba.
In July 2001, Ahounta Djimdoumalbaye, then a pale-ontology student at the University of Poitiers, France, was scouring the desolate, but fossil-rich, Djurab Desert in Central Chad—thousands of miles from the Great Rift Val-ley—and discovered a stunningly complete but distorted skull, along with some isolated jaws and teeth. Revealed through publication a year later by a team led by French paleontologist Michel Brunet, these roughly 7-million-year-old fossils were declared the earliest-known hom-inin species, Sahelanthropus tchadensis. In less than two years, our science went from having essentially no fossils from the critical time range to having three new members at the trunk of the human family tree. But what do these fossils reveal about the earliest members of the human lin-eage? Indeed, how do we know they are on our lineage and not the branch leading to chimpanzees, gorillas, or even an extinct side group of apes with no modern descendants?
On the whole, Ardipithecus, Orrorin, and Sahelan-thropus would look and behave much more like an African ape than a human. The Sahelanthro-pus skull has a thick and projecting brow ridge, a
gorilla-like face, attachments for strong neck muscles, and a braincase the size of an orange (comparable to that of a chimpanzee). Orrorin and Ardipithecus have curved digits and powerfully built arms, which indicate that our earliest
Recently discovered
fossils exhibit puzzling
combinations of humanlike
and ape-like features.
10-12 NH Desilva 918.indd 10 8/6/18 12:13 PM
11September 2018 natural history
relatives were quite comfortable climbing in the trees. But, unlike modern great apes, these ancient hominins walked on two legs and had more humanlike teeth.
What is the evidence for upright walking in these early hominins? A single toe bone from Ar. kadabba had an up-ward tilt at its base that suggested the animal could push off the ground during bipedal walking as humans do. An upper leg bone (femur) from Orrorin had humanlike at-
tachments for the hip muscles that, in humans, facilitate balancing on one leg at a time during bipedal walking. And in Sahelanthropus, the hole in the base of the skull was more forwardly positioned than in great apes, and angled in such a way that the spinal cord would exit out of the bot-tom of the skull—as it does in humans—rather than toward the back, as it does in animals that move on all fours. These anatomies hint at the very beginnings of up-right walking.
Additional evidence that these fossils are early members of our lineage can be found in their mouths. In great apes, the upper canine tooth is large (especially in males) and overhangs the bottom row of teeth. When apes close their mouths, the backside of the upper canine slides against a bicuspid tooth in the lower jaw, honing it in the process. When we close our mouths, our top canine tooth hits the bottom canine tooth and the tips of these teeth wear down as we age. Because large, sharp ca-nines are found in male primates that are polygy-nous (males seek to mate with more than one fe-male), many have related
the reduction in canine size in humans to a shift toward more pair bonding in our lineage, though others attribute it to dietary changes. Either way, in Sahelanthropus, the tip of the canine was worn down, as in humans. And in Orrorin and Ardipithecus, the canines were on the small side and had a reduced honing capacity.
While the canine tooth was getting smaller in these early hominins, the molar teeth were getting bigger and more
Reconstruction by paleoartist Jay Matternes of an Ardipithecus ramidus group based on fossil finds shows that their locomotion included both climbing and bipedal walking.
thickly enameled. This pattern likely reflected dietary changes, as these earliest hominins confronted new envi-ronments. Reconstructions of the habitats of Sahelanthro-pus, Orrorin, and Ardipithecus reveal a mosaic of patchy forests, swamps, lakeshores, and woody grasslands. The earliest hominins appear not to have inhabited an open savanna as once thought; nor did they inhabit a tropical forest. They lived in a mixed environment, with many options. It was here that our unique bipedal locomotion began to be favored—for reasons yet to be satisfactorily explained—and here that our diets shifted away from one dominated by soft, ripe fruit.
But as so often happens in science, there is healthy skepti-cism toward the interpretation of these fossils. Some schol-ars think that Sahelanthropus may be more closely related to gorillas and Ardipithecus to chimpanzees, and therefore neither of them (or at most only one of them) was a hom-inin. Still others have suggested that Sahelanthropus, Or-rorin, and Ardipithecus are all variations of the same hom-inin and that they should all be called Ardipithecus, since that name was coined first (in 1995). Others have posited that perhaps these are not hominins at all, but are branches of ancient apes that evolved hominin-like characteristics (small canines and occasional bipedal walking) in parallel to the lineage that led to us—hominin imposters, in a sense.
We should neither be surprised, nor frustrated, by our inability to securely establish the identity of these ancient remains. Fossils do not come with labels, and the closer in time a fossil gets to the common ancestor of humans and the African apes, the more difficult it becomes to differen-tiate between extinct members of the human, chimpanzee, and gorilla lineages. In fact, fossils such as Sahelanthropus, Orrorin, and Ardipithecus—with combinations of human-like and ape-like features—are precisely what evolutionary theory would predict.
It is also true that, for some time, all the claims and coun-terclaims were based on a collection of fossils that could fit comfortably in a single grocery bag. Then, in 2009, the eagerly anticipated Ardipithecus ramidus skeleton, first
mentioned in print in 1995, was introduced to the world in a collection of research articles published in the journal Sci-ence. A spectacular 4.4-million-year-old skeleton, “Ardi” is destined to be the subject of intense study and debate for decades. Based on the individual’s relatively small canine teeth, Ardi is judged to have been a female. Her brain was small—on the low end of the female chimpanzee range. But in contrast to chimpanzees, her canine teeth were small and blunt, and her molars were larger and more thickly enam-eled. The upper part of her reconstructed pelvis exhibits features that in humans are important for bipedal walking, but the lower features are ape-like and useful for climbing. This pattern is seen in the foot as well—the outside of the foot was humanlike for bipedal push-off, but the big toe
stuck out to the side and could have been used to grasp tree branches [see illustration on previous page].
Ardipithecus ramidus looked to some like a transitional animal between a more ape-like last common ancestor and the more humanlike Australopithecus fossils. But Tim White and his colleagues offered a radically different interpretation of Ardi. They proposed that we have been thinking of human evolution all wrong. Instead of our hav-ing evolved from a chimpanzee-like ancestor, perhaps the great apes are the ones that are highly specialized and have independently changed from a more generalized form. Perhaps the anatomies we have interpreted in humans as being uniquely evolved—such as long, flexible lower backs that help us stand upright—are actually primitive and haven’t changed much in millions of years. To be sure, some anatomies, like our large brains, have changed quite a bit. But, in general, to White and his colleagues, the ver-sion of human evolution in which the chimpanzee slowly and gradually stands up and evolves into the human is not only oversimplified, it is flat out wrong.
Once again, and to no one’s surprise, not every scholar agrees. Some think Ardi’s anatomy is perfectly consistent with hominins having had a chimpanzee-like ancestor. Still others regard Ardi as a hominin imposter, having evolving humanlike features in parallel to the true human lineage. Obviously, more fossils are needed! For one thing, we sorely lack fossils of ancient chimps or gorillas (in other words, “Lucy” equivalents for our great ape cousins), which would show how much their ancestors have evolved in the last seven million years. As for Ardipithecus, the discovery of fossil vertebrae, for instance, would show whether Ardi had a long, humanlike spine or a short, ape-like lower back. A shoulder would help clarify how she climbed in the trees. And a knee would help resolve just how bipedal she was.
Fortunately, a different research team has discovered a second Ardipithecus skeleton in the Gona region of Ethio-pia. Preliminary study of the find, reported in April 2018 at the 87th Annual Meeting of the American Association of Physical Anthropologists by team leader Scott Simpson, of Case Western Reserve in Cleveland, reveals an animal better adapted for upright walking than the original Ardi. These new fossils demonstrate that there was variation in walking ability in Ardipithecus—the raw material from which natural selection would eventually evolve a more skilled walker in Australopithecus.
Jeremy M. DeSilva specializes in the locomotion of the first apes and early members of the human lineage. His particular expertise on the human foot and ankle has contributed to our understanding of the origins and evolution of upright walking in the human lineage. An associate professor in the Department of Anthropology at Dartmouth College in Hanover, New Hampshire, DeSilva has studied wild chimpanzees in western Uganda and early human fossils in museums throughout eastern and southern Africa.
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