Ardipithecus ramidus, Ardi

Wikipedia/T. Michael Keesey

A single, unforgettable image comes to mind when we ponder human origins: a crouching ape slowly standing and morphing into a tall, erect human male poised to conquer every bit of habitable land on this planet. We walk this earth—we, this unparalleled experiment in evolution—reflexively assuming we are the crown of creation. Certainly we are rare and strange: As biological anthropologist Owen Lovejoy of Kent State University says, “The chances that a creature like us will ever happen again are so small that I can’t even measure them.”

But that ascent-of-man picture is looking as dated as the flat earth. A series of scientific and technological breakthroughs have altered much of our fundamental understanding of human evolution. In the new view, the path to Homo sapiens was amazingly dilatory and indirect. Along the way, our planet witnessed many variations on the human form, multiple migrations out of Africa, interspecies trysts, and extinctions that ultimately wiped out all hominid species except one. “Human evolution used to seem simple and linear,” says paleoanthropologist William Jungers of the State University of New York at Stony Brook. “Now, you look at almost any time slice and you see diversity. We may be special and we may be lucky, but we’re far from the only human experiment.”

Unexpected fossil finds keep showing us an ever-expanding variety of human and prehuman species. Probably the most stunning of these recent discoveries is Ardipithecus ramidus, an ancestor who displayed a fantastical mosaic of ape and human traits. 
A. ramidus apparently climbed trees but also walked upright some 4.4 million years ago—more than half a million years before the long-accepted origin of bipedalism.


Our ideas about later human evolution, meanwhile, have been shattered by the remains of a tiny, novel human species with a small but intricately folded brain. Called Homo floresiensis and nicknamed the “hobbit” people, this species found in Indonesia rewrites the scientific story of how humans migrated out of Africa and came to populate the whole world. The hobbits overlapped in time and space with Homo sapiens, showing that even in relatively recent history more than one human species shared our planet—a situation evocative of the colorful world of J. R. R. Tolkien’s Middle-earth, but undeniably real.

The emerging field of paleogenetics has brought perhaps the most surprising news of all. Using DNA sequencing, scientists have learned that anatomically modern humans interbred with Homo neanderthalensis, or the Neanderthals, probably around 60,000 years ago in the Middle East, before they fanned out to populate Europe and Asia. We not only shared the planet with our cousins but shared our DNA as well. Today 1 to 4 percent of the genome for populations living outside of Africa is Neanderthal. A similar form of genetic analysis has also just revealed an entirely new human group, previously unknown: the Denisovans, cousins to Neanderthals. All we have of them so far is a fragment of a 50,000-year-old pinkie finger and most of a molar, found in a cave in Denisova, Siberia. But those fragments were enough to determine that humans living in New Guinea today carry nearly 5 percent Denisovan DNA.

As anthropologists use all the latest tools—genomics, computer analysis, and increasingly sophisticated imaging—to extract deep secrets from the latest fossil finds, they are replacing the “ascent of man” with a captivating new picture of the human family. It edges us decisively closer to understanding not only where we came from but also what made us so much more successful than other, superficially similar primates. “Our relatives, the gorillas and chimpanzees, are still living in the forest in a little piece of West Africa,” Lovejoy says, “and orangutans have survived on two islands in Southeast Asia, but we have evolved rapidly and are everywhere.”

Why, after so many human experiments, are we the only ones left standing?

Ardi, the Human Mosaic


One tantalizing answer comes from an international team of dozens of scientists, including Lovejoy and paleoanthropologist Tim White of the University of California, Berkeley. (The two are the Batman and Robin of the paleoanthropological world, envied and resented for their daring deeds.) Their most notable find is the remarkably complete remains of a 4.4-million-year-old young adult female Ardipithecus ramidus, a creature nicknamed Ardi, which they announced in the journal Science in 2009. Along with more than 100 other fossils representing nearly 40 other Ardipithecus individuals, Ardi was discovered in the scorched landscape of Ethiopia’s Afar Rift, a place where torrential rains regularly wash up traces of ancient stone and bone from different eras. “The rift is our time machine,” says White, who has been working in the area since 1981. “It’s as if Mother Nature is revealing our roots with every rainstorm. We now have hominid remains dating as far back as 5.7 million years and as recently as 80,000 years. We see the deep past here through narrow temporal and spatial windows—walk a mile in any direction and you are either hundreds of thousands of years earlier or later because you are walking on eroding sediments from different slices of time.”

By painstakingly reassembling partially crumbled or smashed specimens, analyzing tooth enamel to determine diet and habitat, and peering inside skeletal bone using powerful high-resolution micro-CT scans (like medical CT scans but with much higher radiation than could ever be used on a patient), the researchers have uncovered what White calls “a complex locomotor hybrid, a creature the likes of which had never been seen.” Translation: Ardi climbed trees with her apelike hands and powerfully built, grasping big toe yet also walked on the ground in her woodland habitat.

Was Ardi a true biped? White and his collaborators looked to the remains of her upper leg and pelvis for clues. After years analyzing digital re-creations of the damaged bones, the group concluded what they had long suspected: The lower part of Ardi’s hip was powerfully primitive, adapted for climbing. In contrast, the upper part of the hip, the ilium, was surprisingly broad—a humanlike adaptation for walking on the ground. As far as Lovejoy is concerned, Ardi is the perfectly logical precursor to Lucy, a small-bodied human ancestor that lived more than a million years after Ardi. For years, Australopithecus afarensis, the species to which Lucy belonged, was regarded as the first truly bipedal hominid. No longer. “Ardi’s adaptations did all the hard evolutionary work for Lucy,” says Lovejoy, an expert on Lucy’s anatomy. “The mosaic Ardi pelvis fits with its equally mosaic foot and reveals, for the first time, how hominids became bipedal.”

For the team at Middle Awash, the part of the rift where Ardi was found, A. ramidus teeth offered another surprise: The males of the species lack the long, fanglike canines that are a hallmark of aggression in apes. Their canines are short and blunt, a signature human trait, one carried over much later by Lucy as well. Lovejoy was startled to see that such a crucial marker of aggression disappeared so early in human evolution. He now suspects this happened because A. ramidus males no longer needed to bare sharp fangs to scare off competing males and ensure female sexual favors. Instead, the males traveled long distances to seek out food for their chosen females, then walked back on their hind legs, carrying provisions in their hands. Bringing females extra food elicited sexual loyalty, and the steady food supply led to reproductive success and expansion on a new scale.