by Kenneth W. Krause.
Kenneth W. Krause is a contributing editor and “Science Watch” columnist for the Skeptical Inquirer. Formerly a contributing editor and books columnist for the Humanist, Kenneth contributes regularly to Skeptic as well. He may be contacted at firstname.lastname@example.org.
She awoke to the cacophony of brightly colored birds and the scent of flesh. Something had died or been killed during the night. She leapt into a tree and, on all fours, deftly palmed her way over a sturdy branch. As she peered downward through the fluttering leaves, she spotted the male who had fathered her last two offspring. He was crouched over a small carcass. She squeaked hopefully and then demandingly as his hungry gaze rose and softened. She descended the tree as he stood upright and watched for others. The male then lumbered a few steps away and turned, offering her what was left of the meat. She sniffed the air greedily, ambling toward it on two legs. Unable to contain herself, she finally screamed in delight as she snatched her prize back into the tree.
In December, Science dubbed her “breakthrough of the year.” Discover ranked her third among the top 100 world-changing discoveries of 2009. Science News called her a “biker chick” and the “ultimate evolutionary party crasher.” Whatever the weighty homage, a truly amazing fossil has turned paleoanthropology on its crusty ear.
Let’s start with the essential facts and figures: First unearthed beginning in 1992, Ardipithecus ramidus and its ancient environment were finally dissected for the world in eleven research articles and authors’ summaries in the October 2, 2009 issue of Science magazine and online. 70 investigators of the international Middle Awash Project—47 of whom became authors—found 135,000 vertebrate pieces and 110 Ar. ramidus specimens comprising at least 36 individuals in Ethiopian sediments dated to 4.4 million-years-old.
“Ardi,” the species’ impish prima donna, is now the most famous and informative fossil hominin since Lucy (Australopithecus afarensis), who in 1974 clocked in at about 3.2 million-years-old. Evidenced by a fairly complete, 125-piece skeleton, Ardi stood 1.2 meters (four feet) tall and weighed about 50 kilograms (110 pounds). Her brain measured only 300 to 350 cubic centimeters in volume—about the same as that of a female chimp.
So why all the fuss? According to Tim “The General” White, lead author and project co-director from the University of California at Berkeley, Ardi’s anatomy offers profound surprises that “no one could have imagined” absent this new evidence. Ar. ramidus also “nullifies” our old, chimpanzee-centric model of hominin origins, adds Kent State University’s C. Owen Lovejoy, and therefore threatens to overturn certain basic assumptions about early human evolution.
The last common ancestor of both chimps (Pan troglodytes) and hominins lived from between five and eight million years ago, depending on whether one emphasizes DNA studies or fossil evidence. Possessing few informative examples of early ape evolution, scientists since Darwin’s time have characterized the ancestor as a sort of proto-troglodyte, with arms adapted for swinging and pelvises and limbs for knuckle walking. The prevailing assumption, in other words, was that humans had done practically all of the evolving since the Pan-hominin split.
The experts have also remained frustratingly vexed by our momentous transition to bipedalism. By the time of Au. afarensis, the hominin pelvis had already developed the essential adaptations for upright walking. Again, the fossil record of earlier human pelvic evolution was scant and mostly uninformative. Nevertheless, anthropologists have long hypothesized that bipedalism was specifically adapted to the savannas and open grasslands occupied by Lucy’s kind and their successors.
But Ardi’s amazing features seem to tell a different story. And so do the project’s collection of more than 150,000 fossilized plant and animal specimens contemporary to Ar. ramidus.
If the last common ancestor didn’t look like an ape, as conventional wisdom maintains it did, then chimps, bonobos, and gorillas must have developed their many similarities independently. This is exceedingly difficult for many scientists to believe. Even so, relative to chimps, Ardi had a short lower muzzle and cranial base, consistent with those of more recent bipeds. And her canines—along with those of her male counterparts—were smaller and less sharpened, or honed, possibly evidencing the adoption among early hominins of less aggressive and more cooperative social and reproductive strategies well prior to the development of enlarged brains and tool use. But more on that below.
Although she retained an ape-like opposable big toe—useful for an arboreal, quadrupedal style of grasping movement over branches called palmigrady—Ardi’s remaining toes were more rigid than those of chimpanzees. And the upper blades of her pelvis were broader as well, more akin to Lucy’s. Such distinctions, say the authors, would have allowed Ardi to balance on one leg and to push off with her back foot. Thus, they infer, Ar. ramidus could actually walk upright on the ground—though much more primitively than Au. Afarensis—as a so-called “facultative” biped.
But not on the savannas or open grasslands, which apparently didn’t even exist in Ardi’s time and place. According to White, Ar. ramidus inhabited a relatively cool and humid woodland analogous to the Kibwezi Forest in present-day Kenya. Among the fossilized remains, the team found no evidence of a humid, closed-canopy tropical rainforest or of sub-desert vegetation typical of the region today. Instead, they discovered wood, seeds, and other plant materials indicating the prevalence of hackberry, fig, and palm trees. They also recovered more than 6000 vertebrate species including many small mammals and birds that were highly sensitive to their environment and, thus, extremely helpful in its reconstruction.
So one might reasonably speculate that Ardi fed more omnivorously than her hyper-specialized ape cousins. The team measured the enamel in more than thirty Ar. ramidus teeth and compared it to those of chimpanzees and later hominins. The enamel in Ardi’s molars, they found, was of intermediate thickness—not so thin as in chimps, which eat soft, ripe, arboreal fruits, but not so thick as in Au. afarensis or later Homo, who consumed more abrasive, terrestrial plants. In the end, the researchers did indeed surmise that Ardi had dined on everything from fruits, nuts, and tubers to insects, small mammals, and bird eggs.
In the Afar tongue, Ar. ramidus refers to a ground-living ape near the root of the family tree. As one might expect, paleoanthropologists aren’t usually shy about claiming the exalted status of human ancestor for their particular specimens. And although White stops short of insisting that his fossils are necessarily ancestral to the Australopithecines—generally accepted as the evolutionary precursors to Homo—his team is quick to emphasize those features that appear to mark Ardi’s kind as a transitional group between the 6- to 7-million-year-old Toumai (Sahelanthropus tchadensis) and Lucy.
Gen Suwa from the University of Tokyo, Japan, for example, writes that despite the diminutive size of Ardi’s skull, “the brain of Ar. ramidus may have already begun to develop some aspects of later hominid-like form and function.” Toumai, Ardi, and Lucy shared short cranial bases and skulls that were downturned at the rear. Again, these features tend to identify them as upright walking hominins rather than apes. Suwa reasons further that the base of Ardi’s brain may have been more “flexed” than in apes, a trait that occurred in Australopithecus along with enlargement of the posterior parietal cortex. In modern humans, this brain region is key to visual and spatial perception.
But Lovejoy envisages an even more profound evolutionary implication in Ar. ramidus. Noting that certain signature human specializations—dentition, locomotion, and encephalization, in particular—are closely related to our reproductive anatomy, Lovejoy suggests that in light of Ardi we can no longer assume that these uniquenesses emerged piecemeal and only since the dawn of the Pleistocene. Rather, he contemplates an interconnected “adaptive suite” of both physiological and behavioral traits that evolved well prior to the age of the Australopithecines “within a unique reproductive strategy that allowed early homini[n]s to thrive relative to their ape relatives and could have ultimately accommodated rapid development of the unusually energy-thirst brain … in Homo.”
Back in 1981, Lovejoy proposed a hominin sexual revolution manifest in Au. afarensis. In most ape species, comparatively large males battle furiously against one another for access to smaller females who openly advertise their sexual receptivity. Not surprisingly, female apes raise youngsters on their own. But the anatomy of Lucy’s kind was quite different; bipedal males were only slightly larger than females and bore small, relatively innoxious canines. From this evidence, Lovejoy inferred that female Australopithecines—which did not advertise their sexual availability—had begun to select more fatherly, monogamous mates that regularly carried food to the mothers of their offspring. Most experts disagreed with the author’s vision of a kinder, gentler ancestor back in 1981, much as they do now.
But Lovejoy remains convinced. Indeed, he now argues that the sexual revolution occurred well before Lucy’s time, even prior to Ar. ramidus. “The evidence is now conclusive,” he declares. Contrary to long-standing wisdom, the loss of large, honed canines among hominins ensued well before they abandoned arboreal habitats and began using tools. “It is far more likely,” he vies, “that our unique reproductive behavior and anatomy emerged in concert with habituation to bipedality and elimination of the [sectorial canine complex].” The ability of Ardi’s kind to exploit both trees and land and to reduce male-to-male conflict, the author summarizes, combined with “three previously unseen behaviors”—regular food carrying, ovulatory crypsis, and pair bonding—to intensify parental investment among males.
Lovejoy’s hypothesis certainly offers an intriguing explanation of why humans developed advanced cognition while chimpanzees, for example, did not. But, unsurprisingly, many of the project’s conclusions have met with tough, even harsh criticism. George Washington University anthropologist, Brain Richmond, emphasizes the folly of conceptualizing the last common ancestor without ever having viewed its fossils. Carol Ward, a specialist in integrative anatomy at the University of Missouri—Columbia, thinks that Ardi’s knees might have been spaced too widely for efficient walking. And Cambridge University anthropologist, William McGrew, claims that in fact there was a sufficient size distinction between Ar. ramidus males and females to indicate heightened male aggression and promiscuity.
I asked William Jungers, anatomist at Stony Brook University in New York, what he thought of the project’s findings. Although eager to examine the fossils and casts for himself before reaching any final conclusions, he calls the existing evidence for Ardi’s bipedality “exceedingly weak (and perhaps nonexistent),” stressing that, technically, chimpanzees qualify as “facultative bipeds” too. He was even less deferential to Lovejoy’s sexual revolution hypothesis, dubbing it a “warmed-over theory of everything.” But Jungers reserves judgment as to just how closely Ardi represents the last common ancestor between hominins and Pan. That conclusion, he says, “awaits a formal, character-based analysis” any number of which should be soon forthcoming.
But everyone seems to agree that White’s team should be applauded for their meticulous efforts and exhaustive treatment of these critical subjects. Yes, we’ve been looking forward to Ar. ramidus for a very long time—more than fifteen years now. But the project’s inspiring results were well worth the wait. The amazing Ardi has challenged and defied anthropologists like no fossil hominin ever has. Because of her, those who crave a deeper understanding of human origins have much to contemplate, and, better yet, a plethora of groundbreaking science to look forward to in the very near future.