Recurring climate changes may have orchestrated where Homo species lived over the last 2 million years and how humankind evolved
Ups and downs in temperature, rainfall and plant growth promoted ancient hominid migrations within and out of Africa that fostered an ability to survive in unfamiliar environments, say climate physicist and oceanographer Axel Zimmerman and colleagues Based on how the timing of ancient climate variations matched up with the comings and goings of different fossil Homo species, the researchers generated a novel — and controversial — outline of human evolution Zimmerman, of Pusan National University in Susan, South Korea, and his team present that scenario April 13 in Nature
Here’s how these scientists tell the story of humankind, starting roughly 2 million years ago By that time, Homo erectus had already begun to roam outside Africa, while an East African species called H erg aster stuck close to its home region H erg aster probably evolved into a disputed East African species called H, which split into southern and northern branches between 850,000 and 600,000 years ago These migrations coincided with warmer, survival-enhancing climate shifts that occur every 20,000 to 100,000 years due to variations in Earth’s orbit and tilt that modify how much sunlight reaches the planet
Then, after traveling north to Eurasia, H possibly gave rise to Dennison around 430,000 years ago, the researchers say And in Central Europe, harsh habitats created by recurring ice ages spurred the evolution of H into Neanderthals between 400,000 and 300,000 years ago Finally, in Southern Africa between 310,000 and 200,000 years ago, increasingly harsh environmental conditions accompanied a transition from H to H sapiens, who later moved out of Africa
But some researchers contend that H, as defined by its advocates, contains too many hard-to-categorize fossils to qualify as a species
An alternative view to the newly proposed scenario suggests that, during the time that H allegedly lived, closely related Homo populations periodically split up, reorganized and bred with outsiders, without necessarily operating as distinct biological species (SN: 12/13/21) In this view, mating among H sapiens groups across Africa starting as early as 500,000 years ago eventually produced a physical makeup typical of people today If so, that would undermine the validity of a neatly branching evolutionary tree of Homo species leading up to H sapiens, as proposed by Zimmermann’s group
The new scenario derives from a computer simulation of the probable climate over the last 2 million years, in 1,000-year intervals, across Africa, Asia and Europe The researchers then examined the relationship between simulated predictions of what ancient habitats were like in those regions and the dates of known hominid fossil and archaeological sites Those sites range in age from around 2 million to 30,000 years old
Previous fossil evidence indicates that H erect us spread as far as East Asia and Java (SN: 12/18/19) Zimmerman’s climate simulations suggest that H erect us, as well as H and H sapiens, adapted to increasingly diverse habitats during extended travels Those migrations stimulated brain growth and cultural innovations that “may have made [all three species] the global wanderers that they were,” Zimmerman says
The new habitat simulations also indicate that H sapiens was particularly good at adjusting to hot, dry regions, such as northeastern Africa and the Arabian Peninsula
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