11.1.8 11.20: How did we get here? The past 200,000 years of human evolution.

About 200,000 years ago, a new human species branched off the Homo ergaster lineage in Africa—the first modern Homo sapiens. Two hundred thousand years is a ballpark figure based on a combination of molecular and fossil evidence. Then, about 60,000 years ago, a small group—probably no more than 100—of these modern humans left Africa and ultimately spread across the earth.

Mitochondrial DNA shows that the initial human migration out of Africa followed three major pathways (FIGURE 11-35). One path turned west after leaving Africa and spread into Europe. A second path turned southeast and spread into southern Asia and through the Indo-Australian Archipelago to Australia. The third migration went northeast, populating northern Asia. This is the group that crossed the Bering Straits bridge about 15,000 years ago and spread southward through the Americas. Even as some groups migrated from Africa, many individuals remained.

There was something about the world that those H. sapiens groups moved into as they left Africa that we would find bizarre—they were not alone. Populations of at least three other species of humans (i.e., species in the genus Homo) were already present in some of the areas that modern humans were moving into. That situation is totally different from the world we know today, in which we are the only human species. What would it be like to live with species that were as similar to us as dogs are to wolves, coyotes, and jackals? How would you interact with an individual who was clearly human, but not human in exactly the same way that you are?

This is the situation that the first modern H. sapiens encountered when they left Africa. Neandertals (H. neanderthalensis) had already spread across Europe and the Middle East. Neandertals were about the same size as modern humans, but more robust and muscular. Fossils of Neandertals often include bones that had been broken and healed. These injuries provide two types of information about Neandertals.

• 1. Neandertals must have lived in organized groups that included a social support system, because these fossils reveal serious injuries. The injured individuals would have been incapacitated for days or even weeks until they healed, and family or clan members must have cared for them during this period. Ritual burials of Neandertals have also been found, and these provide additional support for the hypothesis that they lived in organized social groups.
• 2. The pattern of injuries found in Neandertal skeletons is distinctive—the only modern counterpart is found in professional rodeo bull and bronco riders, people who come in very close contact with large, angry animals. Neandertals probably hunted large mammals (bison, mammoths, wooly rhinoceroses) with short spears that were used for close-up jabbing instead of being thrown from a safe distance.

The H. sapiens who migrated through the Indo-Australian Archipelago encountered two species of humans: H. erectus and H. floresiensis. Homo erectus had migrated to Asia at least a million years earlier and was still present on the island of Java when modern humans arrived, about 50,000 years ago. Although H. erectus was the same size as modern humans, it had a smaller brain—an average brain volume of just over a quart (about 1,000 cc) for H. erectus compared with a quart and a half (about 1,400 cc) for H. sapiens. Nonetheless, H. erectus had technological skills. These may have been the first humans to use fire (the evidence on this is not definitive), and they almost certainly built boats that allowed them to move along coasts and from island to island.

In 2003, researchers discovered stone tools and fossils of a species of human only 3 feet (about 1 m) tall and with a brain volume of just over a pint (about 350 cc) on Flores Island, which lies east of Java. The paleontologists who discovered this species gave it the scientific name Homo floresiensis (“Flores man”), but the world press promptly called it the Hobbit because of its tiny size. When the dwarf human H. floresiensis lived, Flores Island was also home to a dwarf elephant, which H. floresiensis may have eaten, and a giant monitor lizard, which may have eaten H. floresiensis.

After modern H. sapiens groups spread into the areas occupied by the three other human species, these three other species disappeared. Neandertals became extinct about 30,000 years ago, H. erectus about 27,000 years ago, and H. floresiensis about 12,000 years ago. Why did these three species vanish? We’re not sure. Previously, researchers thought that modern H. sapiens must have exterminated the other species, either by monopolizing access to food and living space or by killing them in battles for food and space. But recent results from DNA sequencing suggest that H. sapiens actually encountered and interbred with Neandertals, just as they emerged from Africa. This conclusion is based on the finding that at least 1% to 4% of the genetic makeup of most modern humans includes Neandertal DNA.

Now we are the only extant human species, a situation that is unique in the evolutionary history of humans. Ever since the first branching event, about four million years ago, multiple, closely related species of humans had coexisted. We are the first human species to be alone.