Before “Lucy,” There Was “Ardi”: A Summary of the 11 Major Papers in Science

In a special issue of Science, an international team of scientists has for the first time thoroughly described Ardipithecus ramidus, a hominid species that lived 4.4 million years ago in what is now Ethiopia. This research, in the form of 11 detailed papers and more general summaries, will appear in the journal's 2 October 2009 issue. Science is published by AAAS, the nonprofit science society.

This package of research offers the first comprehensive, peer-reviewed description of the Ardipithecus fossils, which include a partial skeleton of a female, nicknamed “Ardi.”

The last common ancestor shared by humans and chimpanzees is thought to have lived six or more million years ago. Though Ardipithecus is not itself this last common ancestor, it likely shared many of this ancestor's characteristics. For comparison, Ardipithecus is more than a million years older than the “Lucy” female partial skeleton of Australopithecus afarensis. Until the discovery of the new Ardipithecus remains, the fossil record contained scant evidence of other hominids older than Australopithecus.

Through an analysis of the skull, teeth, pelvis, hands, feet, and other bones, the researchers have determined that Ardipithecus had a mix of “primitive” traits, shared with its predecessors, the primates of the Miocene epoch, and “derived” traits, which it shares exclusively with later hominids. Because of its antiquity, Ardipithecus takes us closer to the still-elusive last common ancestor. However, many of its traits do not appear in modern-day African apes. One surprising conclusion, therefore, is that it is likely that the African apes have evolved extensively since we shared that last common ancestor, which thus makes living chimpanzees and gorillas poor models for the last common ancestor and for understanding our own evolution since that time.

This special issue of Science includes an overview article, three articles that describe the environment Ardipithecus inhabited, five that analyze specific parts of Ardipithecus' anatomy, and two that discuss what this new body of scientific information may imply for human evolution. Altogether, 47 different authors from around the world contributed to the total study of Ardipithecus and its environment. The primary authors are Tim White of the University of California, Berkeley; Berhane Asfaw of Rift Valley Research Service in Addis Ababa; Giday WoldeGabriel of Los Alamos National Laboratory; Gen Suwa of the University of Tokyo; and C. Owen Lovejoy of Kent State University.

Press summaries for each article follow:

An Overview of Ardipithecus

(White)

This special collection of Science articles begins with an overview paper that summarizes the main findings of the research effort. Tim White and his coauthors introduce their discovery of over 110 Ardipithecus specimens recovered from 4.4 million-year-old sediments on the floor of the Afar Rift of northeastern Ethiopia, including a partial skeleton with much of the skull, hands, feet, limbs and pelvis. This individual, “Ardi,” was a female who weighed about 50 kilograms and stood about 120 centimeters tall. Until now, researchers have generally assumed that chimpanzees, gorillas and other modern African apes have retained many of the traits of the last ancestor they shared with humans - in other words, that this ancestor was much more chimp-like than human-like. For example, it would have been adapted for swinging and hanging from tree branches, and perhaps walked on its knuckles while on the ground. Ardipithecus challenges these assumptions, however. These hominids appear to have lived in a forested environment, where they climbed on all fours along tree branches - as some of the Miocene primates did - and walked, upright, on two legs, while on the ground. They do not appear to have been knuckle-walkers, or to have spent much time swinging and hanging from tree-branches, especially as chimps do. Overall, the findings suggest that hominids and African apes have each followed different evolutionary pathways, and we can no longer consider chimps as “proxies” for our last common ancestor.

“Ardi's” Environment

(WoldeGabriel, Louchart, White)

The researchers of the Middle Awash project, as this research effort is known (because it is along the middle stretch of the Awash River in Ethiopia's Afar Rift), have unearthed many other animal and plant fossils in addition to the Ardipithecus fossils, which indicate that “Ardi” and her kin lived in a wooded environment. These findings challenge a widespread though controversial idea that bipedalism evolved as hominids adapted to grasslands, since Ardipithecus did walk on two legs, although not as efficiently as later hominids.

Three articles in this package describe “Ardi's” environment in detail. Giday WoldeGabriel and coauthors report on the local geology of the Middle Awash study area, which lies in a region where volcanic activity has left widespread deposits that allow accurate radiometric dating of the fossils. The Ardipithecus fossils were recovered from a rock unit containing beds of silt and clay deposited on a floodplain and that is sandwiched between two key volcanic markers that are each dated to 4.4 million years ago, indicating that the fossils are also that age. Fossilized wood, seeds and other plant materials record the presence of hackberry, fig and palm trees. The authors conclude that this environment was humid and cooler than it is today and contained grassy woodland with forest patches.

In another article, Antoine Louchart and colleagues describe the smaller animals found at the site, which also support the idea this area was a temperate woodland. These include 29 species of birds, primarily small ones like doves, lovebirds, mousebirds, passerines and swifts, as well as several bird species new to science, and up to 20 new species of small mammals, including shrews, bats, rodents, hares and small carnivores.

In a third article, Tim White and colleagues describe the larger animals, which include three primates, the rarest of which is Ardipithecus, whose 109 catalogued fossils collected from between the dated horizons represent a minimum of 36 Ardipithecus individuals. The other two primates that are abundant are baboons and colobine monkeys. Spiral-horned antelopes, thought to be browsers rather than grassland-dwelling grazers, were also abundant.

Teeth Tell a Story of Social Systems and Diet

(Suwa)

The teeth and skull of Ardipithecus are quite different from those of both Australopithecus and modern apes. One important trait, described in an article by Gen Suwa and colleagues, is that the upper canine teeth are more like the stubby ones of modern humans than the long, sharp, pointed ones of male chimpanzees and most other primates. In these species, these dagger-like teeth are continuously sharpened by wear against the lower teeth (the pair of upper and lower teeth is called the “C/P3 complex”), and they are used as slicing weapons in social conflicts. Darwin hypothesized that the C/P3 complex was reduced when our ancestors began using hand-held tools. This idea has been challenged by recent discoveries, however, and the new Ardipithecus findings further overturn it, since they show that this reduction occurred much earlier in hominid evolution than Darwin and other researchers have proposed. The reduced C/P3 also suggests Ardipithecus was less socially aggressive than living chimpanzees and other African apes, the authors say. Furthermore, the fact that the male and female canine teeth are relatively similar in size (again, unlike those of African apes) also suggests that these hominids may have participated in a social system that involved less competition among males. An analysis of the tooth enamel, also reported in this paper, indicates that “Ardi” and her kin had a relatively diverse, omnivorous diet, including fruit and other, woodland-based foods such as nuts and leaves.

An Unusually Complete Skull

(Suwa)

A second article by Gen Suwa and colleagues looks at the Ardipithecus skull, which is relatively complete in the “Ardi” specimen, despite being badly crushed. Because the bones were so fragile and deformed, the researchers assembled the fragments into more than 60 key pieces of the braincase, face and teeth, allowing them to digitally reconstruct a largely complete cranium and lower jaw. The results indicate that “Ardi” had a small brain, even smaller than that of Australopithecus and similar to that of bonobos and female chimpanzees. Her face had a projecting muzzle, giving her a decidedly ape-like appearance, although it didn't thrust forward quite as much as the lower faces of modern African apes do. And some features of her skull, such as the ridge above the eye socket, are quite different from those of chimpanzees. The details of the bottom of the skull, where nerves and blood vessels enter the brain, indicate that “Ardi's” brain was positioned in a way more similar to that of Australopithecus and modern humans, possibly suggesting that the hominid brain may have been already poised, 4.4 million years ago, to expand regions of the modern human brain involved in aspects of visual and spatial perception.

“Ardi's” Arms and Hands

(Lovejoy)

The hand and wrist of Ardipithecus are a mosaic of primitive traits, which this hominid shared with its distant very early ape ancestors, and few new ones, but they don't include the hallmark traits of the modern tree-hanging, knuckle-walking chimps and gorillas, C. Owen Lovejoy and colleagues report. In contrast to the rest of the fossil record, which includes few hominid hand bones, the hands of the “Ardi” skeleton are virtually complete and intact. They show that “Ardi” had long fingers, like other apes, but the joints of her wrists and those between her relatively short palms and fingers were flexible, allowing her to support her body weight on her palms while moving along tree branches. In contrast, these joints are stiffer in chimps and gorillas, which routinely support their upper-body weight on their knuckles. “Ardi's” hands also lack many of the other specializations that protect chimp and gorilla hands from injury while they climb and feed in trees. The researchers thus conclude that Ardipithecus was a careful climber, and that the anatomical features that allowed modern-day African apes to swing, hang, and easily move through the trees must have emerged only after each African ape separated from its shared ancestor with hominids. Lovejoy and his coauthors also propose that our earliest ancestors' hands were already relatively dexterous, requiring only a slight enlargement of thumbs and shortening of fingers to eventually use and make tools.

The Ardipithecus Pelvis and Upright Walking:

(Lovejoy)

The Ardipithecus pelvis is particularly useful for understanding how the hominid skeleton became progressively modified for bipedality, according to another article by C. Owen Lovejoy and coauthors. Ardipithecus' feet and hands show that this hominid did spend time in trees, and in some ways its pelvis was more ape-like than that of Australopithecus. But, certain features also allowed the pelvis to support two-legged walking. For example, the shape of the hipbone, or ilium, including the presence of a special growth site unique to hominids, known as the “anterior inferior iliac spine,” indicate that Ardipithecus' gluteal muscles were positioned so that “Ardi” could walk without shifting her center of mass from side to side. However, the lower pelvis and femur was still almost entirely ape-like, presumably because Ardipithecus still required massive hindlimb muscles for active climbing. Ardipithecus could also run, but probably with less speed and efficiency than humans.

Feet for Both Grasping and Striding:

(Lovejoy)

Ardipithecus' feet were rigid enough to support them while walking bipedally but were also endowed with opposable, grasping big toes that would have been useful for climbing, according to a third article coauthored by C. Owen Lovejoy and colleagues. The “Ardi” skeleton includes most of the foot and includes a specific bone called the os perineum that turns out to be pivotal to our understanding of foot evolution. This bone, which is embedded within a tendon, facilitates the mechanical action of the primary muscle that draws in the big toe during the grasping motion. Scientists have known little about this bone except that it was present in Old World monkeys and gibbons but generally not in our more recent ape relatives. (African apes also have feet well-adapted for grasping, of course, but their feet have evolved to become more flexible and compliant to the substrate.) Now, we know that “Ardi” possessed this bone, suggesting that the last common ancestor to African apes and humans had a more monkey-like foot than an ape-like one. “Ardi's” foot was also monkey-like in its rigidity. Monkeys need rigid feet for leaping between trees. For Ardipithecus, this rigidity made walking possible. For example, it allowed the hominid to push off its back foot and step onto its front foot. Overall, the Ardipithecus feet, and presumably those of our last common ancestor with the African apes, were relatively unspecialized and did not much resemble that of chimps and gorillas, the authors conclude.

“Ardi,” “Lucy” and Our Last Common Ancestor with Chimpanzees:

(Lovejoy)

Until now, Australopithecus afarensis - whose most well-known representative is the individual “Lucy” - has often been considered the best stand-in for our last common ancestor with chimpanzees. It's now clear, however, that Ardipithecus, more than a million years older than Lucy, was more similar to this still unrecovered ancestor, and represents a more primitive species in the hominid lineage, according to C. Owen Lovejoy and coauthors. They also argue that some of the traits that “Lucy” and modern humans share with African apes, such as more upright carriage of the upper body, actually evolved separately in the hominid and African-ape lineages. Australopithecus, which lived 1-4 million years ago, does have many primitive characteristics, including its small brain-size, which has roughly the same proportion to its body size as that of the modern African apes. The fact that the genomic similarities between modern humans and chimps were coming to light at the same time as Australopithecus afarensis was being recovered and described probably also influenced our tendency to think of “Lucy” as representative of the last common ancestor to humans and chimpanzees (or, the “LCA,” as anthropologists say).

Instead, the new findings on Ardipithecus indicate that the LCA was very different from specialized modern African apes. Its feet functioned only partly like those of apes and much more like those of living monkeys and Early Miocene apes such as Proconsul. Its lower back was mobile and probably had more vertebrae than the stiff backs African apes have. It appears that the LCA's hand was surprisingly unique. Not only was its thumb musculature robust (unlike that of an ape), its midcarpal joint allowed the wrist to bend backward to a great degree, still seen to facilitate Ardipithecus' movement on its palms along tree branches. None of the changes that apes have evolved to stiffen their hands for hanging and vertical climbing were present, so the LCA's locomotion probably did not resemble that of any living ape, according to Lovejoy and coauthors. The LCA's descendant, Ardipithecus, became bipedal by modifying its upper pelvis without yet abandoning its grasping big toe. The authors write that the Ardipithecus “is so rife with anatomical surprises that no one could have imagined it without direct fossil evidence.”

Reexamining Human Origins:

(Lovejoy)

Ardipithecus illuminates human origins because it clarifies our relationship to Australopithecus, says C. Owen Lovejoy in the final article. He elaborates on some of the points raised in the previous articles, about the absence of a projecting, sharp canine tooth in Ardipithecus males, and the relatively similar body size of males and females in this early hominid. These characteristics suggest that male-male conflict was less prevalent than it is among other primates like chimpanzees and gorillas. What advantages might these traits have offered the hominid lineage? On a similar note, upright walking carried no energetic advantage for Ardipithecus, which lacked the specializations evolved in later hominids such as Australopithecus. So, what benefits might bipedalism have offered, even during this early stage of hominid evolution? Lovejoy proposes a scenario in which these traits and others make up an “adaptive suite,” a web of interrelated characteristics that influence an organism's reproductive strategy. He says that if we combine what we know about mammalian (including human) reproductive physiology and the hominid fossil record, it appears that a major shift in life-history strategy was involved in the emergence of early humans. That shift likely combined three new behaviors that would have developed in response to the ability to exploit both trees and the land surface, and in response to reduced male-male conflict. These behaviors are: regular food carrying, pair-bonding between males and females, and reproductive “crypsis,” meaning females did not advertise ovulation (as, for example, chimpanzees do). Together, Lovejoy says, these behaviors would have substantially intensified male parental investment among early hominids - a “breakthrough” adaptation with major consequences for early hominids and all their descendants, including ourselves.

Kathy Wren

30 September 2009