Neanderthals

Homo neanderthalensis emerged about 200,000 years ago as a regional species or ecotype of Homo heidelbergensis in Europe. A parallel species, Homo sapiens, was developing at the same time in east Africa. Neanderthals evolved into thee subgroups: one in Asia (blue on the map below), one in northern Europe (pink), and one in southern (rust). The boundary between east and west lay in the Caucasus. The last two were divided by the Alps, which were covered at times with glaciers during the Pleistocene.

Anders Götherström’s team found differences between the eastern and western groups increased about 48,000 years ago. The DNA from bones found before that period and those found in the east contained the range of genetic variation one would expect. Those from the west showed much less: .0063 compared with an index of .0191 for eastern Neanderthals and .0270 for species in Africa.

The reduced variation recorded a demographic crisis that occurred when the western population had been severely reduced in size and regrown from a single group or area. Thomas Schmitt has noted that when species expanded from Pleistocene refuges they exhibit "only weak genetic differentiation during the process of range expansion."

Götherström’s group noted the break in Neanderthal genetic continuity occurred after ice bergs breaking from the Greenland ice sheet added cold, fresh water to the north Atlantic. The altered ocean currents brought frigid conditions to western Europe.

Hartmut Heinrich was the one who first observed the changes in ocean core sediments that occurred when ice bergs calved. The fifth such episode has been dated to 50,000 years ago by Gerald Bond’s team. Mark Maslin’s group thought the rafting was fairly rapid and each episode probably lasted less than a thousand years. That’s still 50 generations that reproduce every 20 years.

Quick change is difficult for plants, animals, and their dependants. Götherström’s colleagues hypothesized the western Neanderthals that survived were those who moved into protected refuges from which they emerged when climatic conditions improved.

During the ice age, there were three major European protected areas for species fleeing the spreading glaciers: the Iberian, Italian and Balkan peninsulas (the R's on the map below). Schmitt has reviewed their ranges, which include small mammals, insects, and plants. Within the Spanish peninsula, he noted one nursery area was in the southwest. The other in the southeast had some connections to the maghreb of northern Africa. The refuges roughly coincide with the distribution of southern Neanderthals.

Notes:
Bond, Gerard, et alia. "Correlations Between Climate Records from North Atlantic Sediments and Greenland Ice," Nature 365:143-147:1993; date for Heinrich event 5.

Fabre, Virginie, Silvana Condemi, and Anna Degioanni. "Genetic Evidence of Geographical Groups among Neanderthals," PLoS One, 15 April 15, 2009.

Götherström, Anders, Thomas P. Gilbert, J. Carlos Díez Fernández-Lomana, Eske Willerslev, and Juan Luis Arsuaga. "Partial Genetic Turnover in Neandertals: Continuity in the East and Population Replacement in the West," Molecular Biology and Evolution 29:1893-1897:2012.

Heinrich, H. "Origin and Consequences of Cyclic Ice Rafting in the Northeast Atlantic Ocean During the past 130,000 Years," Quaternary Research 29:142-152:1988; cited by Wikipedia. The episodes are referred to as Heinrich Events 5 and 6.

Schmitt, Thomas. "Molecular Biogeography of Europe: Pleistocene Cycles and Postglacial Trends," Frontiers in Zoology, 2007.

Graphics:
1. Delamaison, Manon. "Distribution Géographique des Sites du Moustérien," uploaded to Wikimedia Commons, 15 October 2013.

2. Neanderthal subgroups, from Fabre, above. Pink represents the range of western Neanderthals, blue the east, and gold the southern.

3. Pleistocene refuges, from Schmitt, above. The R’s represent refugia, the H’s diffusion paths for recolonization.