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.
