The fossils found in Yunnan province, China.Photo: Reuters
The climate change that occurred about 34 million years ago may hold the reason as to why humans originated in Africa and not in Asia. More specifically, the deterioration of climate marking the Eocene-Oligocene Transition (which occurred about 34 million years ago) could have acted as an evolutionary filter, allowing different types of primates to evolve in Africa as compared to Asia. The ensuing dominance of the subset of primates known as anthropoids in African regions must have led to the evolution of humans there, according to a paper in Science. Therefore, even though the earliest primate fossils have been found in Asia, the actual evolution of humans took place in Africa.
Geological calendar
The geological calendar relating to the evolution of primates and humans encompasses the so-called periods of which two are of interest here — the Paleogene and the Neogene. The Paleogene, which lasted over 43million years, beginning 66 million years ago (mya) and ending about 23 mya, was the time when mammals evolved from simpler forms. This was followed by the Neogene period (starting from the end of Paleogene to about 2.5 mya) during which time early humans evolved.
The Paleogene itself is divided into three epochs: the Paleocene, Eocene and Oligocene.
Nearly 55 mya, the earth suffered a global increase in temperature of about 5 degrees Celsius. This is known as the Paleocene-Eocene Thermal Maximum. During this, the primates, from a subset of which humans eventually evolved, being highly sensitive to temperature changes, migrated from their place of origin in lower latitudes, to the northern land regions. Subsequently, temperatures fell during the Eocene-Oligocene Transition. Marking this was a deterioration of climate which caused these primates to again retreat to lower latitudes.
Asian anthropoids
In a paper published in Science, Xijun Ni and coworkers study a recently discovered set of fossils of primates from the early Oligocene period found in the Yunnan province of southern China. These fossils point to an African origin of humans in the Neogene period, even though anthropoids originated in Asia.
“The global climate deterioration, which happened 34 million years ago, changed the fate of anthropoids. We suggest that this global climate deterioration is an evolutionary filter for primates,” says Xijun Ni in an email to this correspondent.
The authors find that of the six fossils studied, only one is an anthropoid. Earlier known records of primate fossils from the Late Eocene period from China, Myanmar and Thailand have shown a domination of stem anthropoids. So the lack of anthropoids in the set of fossils discovered now, which date later geologically, could be attributed to the filtering mechanism caused by lowering of temperature and deterioration of climate that marked the Eocene-Oligocene Transition, the authors infer.
Dr Ni clarifies, “In Asia, this [evolutionary] filter removed most of the anthropoids but left lemur-like primates. In Africa, the effect of this filter is on the opposite side. It removed most of the lemur-like primates but gave anthropoids more opportunities. After the filtering, the evolutionary center of anthropoids moved from Asia to Africa.”
They also consider a fossil collection from Pakistan, dated at a late early-Oligocene and find evidence of a few primates. However, these were identified as belonging to those species of small-bodied primates which did not evolve into humans later.
In a complementary manner, the late Eocene-early Oligocene primates from the Afro-Arabian region had shown a very different response to the Eocene-Oligocene Transition. There the anthropoids diversified “taxonomically and ecologically,” leading to the eventual evolution of humans in the Neogene period.
