How ancient, recurring climate changes may have shaped human evolution

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 Timmermann 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. Timmermann, of Pusan National University in Busan, 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. ergaster stuck close to its home region. H. ergaster probably evolved into a disputed East African species called H. heidelbergensis, 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. heidelbergensis possibly gave rise to Denisovans around 430,000 years ago, the researchers say. And in central Europe, harsh habitats created by recurring ice ages spurred the evolution of H. heidelbergensis into Neandertals 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. heidelbergensis to H. sapiens, who later moved out of Africa.

But some researchers contend that H. heidelbergensis, 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. heidelbergensis 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 Timmermann’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. erectus spread as far as East Asia and Java (SN: 12/18/19). Timmermann’s climate simulations suggest that H. erectus, as well as H. heidelbergensis 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,” Timmermann 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.

Climate, habitat and fossil data weren’t sufficient to include additional proposed Homo species in the new evolutionary model, including H. floresiensis in Indonesia (SN: 3/30/16) and H. naledi in South Africa (SN: 5/9/17).

It has proven difficult to show more definitively that ancient environmental changes caused transitions in hominid evolution. For instance, a previous proposal that abrupt climate shifts resulted in rainy, resource-rich stretches of southern Africa’s coast, creating conditions where H. sapiens then evolved (SN: 3/31/21), still lacks sufficient climate, fossil and other archaeological evidence.

Paleoanthropologist Rick Potts of the Smithsonian Institution in Washington, D.C., has developed another influential theory about how climate fluctuations influenced human evolution that’s still open to debate. A series of climate-driven booms and busts in resource availability, starting around 400,000 years ago in East Africa, resulted in H. sapiens evolving as a species with a keen ability to survive in unpredictably shifting environments, Potts argues (SN: 10/21/20). But the new model indicates that ancient H. sapiens often migrated into novel but relatively stable environments, Timmermann says, undermining support for Potts’ hypothesis, known as variability selection.

The new findings need to be compared with long-term environmental records at several well-studied fossil sites in Africa and East Asia before rendering a verdict on variability selection, Potts says.

The new model “provides a great framework” to evaluate ideas such as variability selection, says paleoclimatologist Rachel Lupien of Lamont-Doherty Earth Observatory in Palisades, N.Y. That’s especially true, Lupien says, if researchers can specify whether climate and ecosystem changes that played out over tens or hundreds of years were closely linked to ancient Homo migrations.

For now, much remains obscured on the ancient landscape of human evolution.