The Earth’s climate has always been in flux. Since our ancestors split off from the primate evolutionary tree millions of years ago, the planet has experienced dramatic shifts between moist and dry periods, as well as long-lasting glacial freezes and thaws. It’s clear that early humans were able to adapt to such changes—our very existence attests to their success. However, an increasing number of scientists believe that major climate shifts may have also shaped some of humanity’s defining characteristics.
An international team of scientists published a study in Nature that provides clear evidence for a link between astronomically-driven climate change and human evolution. The team of experts in climate modeling, anthropology, and ecology was able to determine which environmental conditions archaic humans likely lived under by combining the most extensive database of well-dated fossil remains and archeological artifacts with an unprecedented new supercomputer model simulating Earth’s climate history of the past 2 million years.
Climate change’s impact on human evolution has long been suspected, but proving it has been difficult due to a scarcity of climate records near human fossil-bearing sites. To avoid this issue, the team investigated what the climate was like in their computer simulation at the times and places where humans lived, according to the archeological record. This revealed the preferred environmental conditions of various hominin groups. The team then searched the model for all the places and times when those conditions occurred, creating time-evolving maps of potential hominin habitats.
Our climate-based reconstruction of hominin lineages is quite similar to recent estimates obtained from either genetic data or the analysis of morphological differences in human fossils, which increases our confidence in the results.
Dr. Jiaoyang Ruan
“Even though different groups of archaic humans preferred different climatic environments, their habitats all responded to climate shifts caused by astronomical changes in earth’s axis wobble, tilt, and orbital eccentricity with timescales ranging from 21 to 400 thousand years,” said Axel Timmermann, lead author of the study and Director of the IBS Center for Climate Physics (ICCP) at Pusan National University in South Korea.
The scientists repeated their analysis, but with the ages of the fossils shuffled like a deck of cards, to test the robustness of the link between climate and human habitats. If the evolution of climatic variables in the past had no impact on where and when humans lived, both methods would produce the same habitats. However, when using the shuffled and realistic fossil ages, the researchers discovered significant differences in the habitat patterns of the three most recent hominin groups (Homo sapiens, Homo neanderthalensis, and Homo heidelbergensis).
“This result implies that at least during the past 500 thousand years the real sequence of past climate change, including glacial cycles, played a central role in determining where different hominin groups lived and where their remains have been found,” said Prof. Timmermann.
“The next question we set out to address was whether the habitats of the different human species overlapped in space and time. Past contact zones provide crucial information on potential species successions and admixture,” said Prof. Pasquale Raia from the Università di Napoli Federico II, Naples, Italy, who together with his research team compiled the dataset of human fossils and archeological artefacts used in this study. From the contact zone analysis, the researchers then derived a hominin family tree, according to which Neanderthals and likely Denisovans derived from the Eurasian clade of Homo heidelbergensis around 500-400 thousand years ago, whereas Homo sapiens’ roots can be traced back to Southern African populations of late Homo heidelbergensis around 300 thousand years ago.
“Our climate-based reconstruction of hominin lineages is quite similar to recent estimates obtained from either genetic data or the analysis of morphological differences in human fossils, which increases our confidence in the results,” remarks Dr. Jiaoyang Ruan, co-author of the study and postdoctoral research fellow at the IBS Center for Climate Physics.
The new study was made possible by the use of Aleph, one of South Korea’s fastest supercomputers. Aleph, located at the Institute for Basic Science headquarters in Daejeon, ran nonstop for over 6 months to complete the most comprehensive climate model simulation to date. “The model generated 500 Terabytes of data, enough to fill several hundred hard disks,” said Dr. Kyung-Sook Yun, the experiments’ lead researcher at the IBS Center for Climate Physics.
“It is the first continuous simulation with a state-of-the-art climate model that covers earth’s environmental history of the last 2 million years, representing climate responses to the waxing and waning of ice-sheets, changes in past greenhouse gas concentrations, as well as the marked transition in the frequency of glacial cycles around 1 million years ago,” adds Dr. Yun.
“So far, the paleoanthropological community has not utilized the full potential of such continuous paleoclimate model simulations. Our study clearly illustrates the value of well-validated climate models to address fundamental questions on our human origins,” says Prof. Christoph Zollikofer from the University of Zurich, Switzerland, and co-author of the study.
Beyond the question of early human habitats and the times and places of human species origin, the research team looked into how humans may have adapted to changing food resources over the last 2 million years.
We discovered an intriguing pattern when we examined the data for the five major hominin groups. Around 2-1 million years ago, early African hominins preferred stable climatic conditions. As a result, they were limited to relatively narrow habitable corridors. Following a major climatic transition around 800 thousand years ago, a group known as Homo heidelbergensis adapted to a much broader range of available food resources, allowing them to become global wanderers, reaching remote regions in Europe and eastern Asia.
