Our genus, Homo, evolved over 3 million years by adapting to increasingly diverse environments. Now, a new study published in Science deeply explores how six species of Homo (H. ergaster, H. habilis, H. erectus, H. heidelbergensis, H. neanderthalensis, and early H. sapiens) adapted to habitats across Africa and Eurasia.
In its analysis, the team of scientists from South Korea and Italy used data from more than 3,000 human fossil specimens and archaeological sites. They then combined those data with climate and vegetation models of the past 3 million years.
Their study found that during the early to middle Pleistocene (about 2.6 million years ago to 0.5 million years ago), massive changes in Earth’s climate played a role in the distribution of vegetation, as well as the evolutionary development of the Homo species studied.
Elke Zeller, a Ph.D. student at Pusan National University, South Korea, and the first author of the new study, explained that the climate has cooled considerably over the past 3 million years. Reasons for this pattern of climate change include a gradual decrease in atmospheric carbon dioxide levels, glacial cycles brought on by long-term changes in Earth’s orbit and axis, and, after the Mid-Pleistocene Transition (MPT) about 1 million years ago, a lengthening of glacial cycles from about 41,000 years to about 100,000 years.
“These long-term climate trends also affected vegetation,” Zeller said. “Because cold air holds less water, planetary cooling was accompanied by an overall drying.”
This global cooling resulted in a shrinking of warm tropical forests in central Africa and southern Europe—habitats to which the early hominins H. ergaster and H. habilis were adapted. Forests were replaced by more open environments such as grassland and dry shrubland. “Especially after the MPT, about 1 million years ago, we see a big shift towards more open vegetation in these areas,” Zeller said.
“The study affirms what has long been thought—that dispersal of hominins occurs against a backdrop of continental-scale expansion of open grassland biomes and climate change…in distinct regions in early-middle Pleistocene period.”
Phil Glauberman, a researcher at the Institute of Human Paleoecology and Social Evolution and Universitat Rovira i Virgili, Tarragona, Spain, said that the “study affirms what has long been thought—that dispersal of hominins within and outside of Africa occurs against a backdrop of continental-scale expansion of open grassland biomes and climate change…in distinct regions in early-middle Pleistocene period.” Glauberman was not involved with the research. José Sebastián Carrión, a professor of evolutionary botany at Murcia University, Spain, added that “perhaps an opening of the landscape would imply fewer plant or animal resources for subsistence, and [early hominins] may have ventured out in search of new resources. Or they may have done so because the open landscape facilitated long-distance visibility.”
Whatever the motivation, it’s a fact that the loss of tree cover is correlated with hominins expanding their range into new areas, said Carrión, who was also not involved in the new research.
Around 2 million years ago, H. erectus ventured into Europe from Africa. Its habitat preference adapted to both open and closed landscapes. “This shift in landscape preference is very interesting because it shows that they could adjust to the different vegetation types we see in Europe,” explained Zeller.
In Europe around this time and throughout the MPT, there was an increase in the latitudinal range of tundra, boreal forests, and grasslands and a reduction in temperate forests. “H. erectus was able to adapt to this diverse landscape and dispersed across Eurasia,” Zeller said, and “subsequently, H. heidelbergensis, the successor species of H. erectus, adapted to colder conditions, moving into boreal forests.”
When H. neanderthalensis came on the scene a little more than 500,000 years ago, large parts of northern Europe and Asia were still experiencing the Ice Age, with big ice sheets, wide swaths of tundra, and cold deserts.
Zeller said that in examining Neanderthal sites, her team “did not see a big fraction…in tundra areas nor in desert areas, leading us to conclude that during colder times, they moved towards southern Europe.” Neanderthals, the team reasoned, had limited capacity to adapt to cold climates and chose to settle in closed landscapes such as temperate boreal forests.
Among the hominins studied, only H. sapiens—our own species—was able to successfully adapt to mosaic landscapes, which include harsh environments such as deserts and tundra.
“The most outstanding conclusion of this paper,” Carrión said, is that “over the evolution of Homo, and particularly our species, H. sapiens, a trend towards exploitation of mosaic landscapes is now more strongly supported by data and simulations.”
Glauberman concurred, saying “Zeller and colleagues hypothesize that H. sapiens’ adaptability to changing climate regimes and extreme environments evolved through consistent occupation of mosaic landscapes as they dispersed throughout the globe during the late Pleistocene.”
He explained that highly diverse ecosystems, with abundant and varied food and material resources, likely contributed to the cultural and technological innovations that enabled H. sapiens to thrive.
—Deepa Padmanaban (@deepa_padma), Science Writer