The development of Homo sapiens and Neandertals is linked to a contested predecessor, according to shifting environments.
Climate change may have impacted where the Homo species lived and how humanity evolved during the last 2 million years.
According to climate scientist and oceanographer Axel Timmermann and colleagues, temperature, rainfall, and plant growth influenced early hominid migrations within and outside of Africa, fostering an ability to live in unknown habitats. The researchers created a fresh — and contentious — sketch of human development based on how the timing of past climate fluctuations matched up with the comings and goings of distinct fossil Homo species. Timmermann and his colleagues from Pusan National University in Busan, South Korea, published their findings in Nature on April 13th.
Here's how these researchers portray the narrative of humanity, which dates back around 2 million years. Homo erectus had already begun to spread outside of Africa at that time, but an East African species known as H. ergaster stayed close to home. Between 850,000 and 600,000 years ago, H. ergaster developed into the disputed East African species H. heidelbergensis, which separated into southern and northern branches. These migrations corresponded with warmer, more conducive to survival climate fluctuations that occur every 20,000 to 100,000 years as a result of differences in Earth's orbit and tilt, which alter the amount of sunlight reaching the planet.
The researchers believe that H. heidelbergensis gave rise to Denisovans around 430,000 years ago after moving north to Eurasia. Between 400,000 and 300,000 years ago, harsh habitats generated by repeated ice ages aided the evolution of H. heidelbergensis into Neandertals in central Europe. Finally, between 310,000 and 200,000 years ago, harsher environmental conditions followed a change from H. heidelbergensis to H. sapiens in southern Africa, which later traveled out of Africa.
However, some experts argue that H. heidelbergensis, as described by its proponents, comprises far too many difficult-to-classify fossils to be considered a species.
Closely related Homo groups frequently split up, restructured, and bred with outsiders during the time that H. heidelbergensis reportedly flourished, without necessarily operating as distinct biological species, according to an alternative view to the newly presented scenario (SN: 12/13/21). According to this theory, mating among H. sapiens tribes across Africa as early as 500,000 years ago resulted in the physical makeup of today's individuals. If this is the case, Timmermann's group's proposal of a cleanly branching evolutionary tree of Homo species leading up to H. sapiens would be invalidated.
The new scenario is based on a computer simulation of the likely climate spanning Africa, Asia, and Europe for the previous 2 million years, in 1,000-year intervals. The researchers next looked at the dates of known hominid fossils and archaeological sites in relation to computer projections of what ancient environments were like in those areas. The age of the sites ranges from 2 million to 30,000 years.
H. erectus is thought to have expanded as far as East Asia and Java, according to previous fossil data (SN: 12/18/19). Climate simulations by Timmermann imply that H. erectus, H. heidelbergensis, and H. sapiens adapted to progressively diversified habitats over long journeys. These travels sparked brain development and cultural advances, according to Timmermann, which "may have made [all three species] the worldwide wanderers that they were."
The new habitat models also show that H. sapiens adapted well to hot, arid environments like northeastern Africa and the Arabian Peninsula.
Additional putative Homo species, such as H. floresiensis in Indonesia (SN: 3/30/16) and H. naledi in South Africa (SN: 5/9/17), were not included in the revised evolutionary model due to insufficient climate, habitat, and fossil data.
It's been difficult to prove conclusively that ancient environmental shifts caused hominid evolution transitions. For example, a previous hypothesis that rapid climate alterations resulted in rainy, resource-rich portions of southern Africa's coast, resulting in conditions conducive to the evolution of H. sapiens (SN: 3/31/21) is still lacking in climate, fossil, and other archaeological evidence.
Rick Potts, a paleoanthropologist at the Smithsonian Institution in Washington, D.C., has proposed another controversial theory regarding how climate variability influenced human evolution. Starting roughly 400,000 years ago in East Africa, a succession of climate-driven booms and busts in resource availability resulted in H. sapiens evolving as a species with a remarkable ability to live in unpredictably shifting circumstances, according to Potts (SN: 10/21/20). The new model, however, suggests that ancient H. sapiens frequently migrated into unfamiliar but rather stable settings, contradicting Potts' notion of variable selection, according to Timmermann.
Before passing judgment on variability selection, Potts says the new data must be compared to long-term environmental records at numerous well-studied fossil sites in Africa and East Asia.
Paleoclimatologist Rachel Lupien of the Lamont-Doherty Earth Observatory in Palisades, New York, says the new model "provides a terrific framework" for evaluating hypotheses like variability selection. That's especially true, Lupien says, if researchers can pinpoint whether tens or hundreds of years of climate and ecosystem change were linked to early Homo migrations.
Much of the old landscape of human evolution remains veiled for the time being.