Icy Isolation May Have Led to New Human Species: Cold-Climate Refuges Possibly Influenced Homo Evolution
Bower, Bruce, Science News
After ancient people left their African homeland, they migrated into Asia and Europe, taking refuge from ice age conditions in areas isolated from other populations, two new reports suggest. That isolation may have prompted the evolution of new Homo species, including a mysterious Asian population dubbed Denisovans and possibly an unusual-looking humanlike group now identified in China.
Ice age asylums "are critical to understanding the expansion of H. sapiens out of Africa, the extinction of Neandertals and Denisovans, and interbreeding between these populations," comments anthropologist Robin Dennell of the University of Sheffield in England.
Fossils unearthed in two caves in southwestern China come from an unusual-looking line of Homo sapiens, or perhaps a previously unknown Homo species, say anthropologist Darren Curnoe of the University of New South Wales in Sydney and his colleagues. This group lived near modern-looking people between 14,300 and 11,500 years ago.
Ancient bones unearthed previously and in new digs at the Chinese caves combine features of people today with flaring cheekbones and other traits of poorly understood African Homo fossils from more than 100,000 years ago. Because this anatomically peculiar population survived alongside modern-looking people until almost 11,000 years ago, Curnoe suspects that the new fossils represent a separate Homo species that originated in Asia.
"We're cautious about classifying these fossils, because scientists lack a satisfactory biological definition of Homo sapiens," Curnoe says. He and his colleagues report the findings online March 14 in PLoS ONE.
Ancient people left Africa as early as 120,000 years ago, so the Chinese fossils might be those of early migrants who evolved in relative isolation for tens of thousands of years without contributing genetically to people today, Curnoe suggests.
Instead, the ancient features of the new Chinese finds might reflect interbreeding with the Stone Age, humanlike species called Denisovans, remarks anthropologist Chris Stringer of the Natural History Museum in London. Denisovans, identified from DNA taken from a single finger bone found in Siberia, interbred with humans in southeastern Asia at least 44,000 years ago (SN: 11/5/11, p. 13). Researchers regard Denisovans as close relatives of Neandertals.
In the March 16 Science, Stringer and evolutionary ecologist John Stewart of Bournemouth University in England propose that the evolution of humans and earlier Homo species that reached Asia and Europe hinged on small populations that took refuge in ecological sanctuaries during recurring ice ages.
As with many plants and animals, climate-induced corralling of Homo groups into restricted habitats prompted the evolution of new species, most notably Neandertals and Denisovans, Stringer and Stewart hypothesize. Their new work lays out a framework of what early human migration and evolution might have looked like.
Consider Homo heidelbergensis, alikely ancestor of Neandertals and H. sapiens that left Africa between 600,000 and 400,000 years ago. H. heidelbergensis apparently survived in livable parts of southwestern Asia during ice ages. A long-isolated H. heidelbergensis population evolved into Neandertals, Stringer suggests.
Neandertals interbred with H. sapiens around 45,000 years ago, as ancient people reached a climate-friendly part of southern Europe already populated by their evolutionary cousins, Stringer proposes. Neandertals then headed to Europe's southwestern corner and died out by 30,000 years ago. …