Fossil finds may explain why humans evolved in Africa, not Asia
KU News Service/Univ. of Kansas
Fossil discoveries in southern China point to an evolutionary crossroads around 34 million years ago that resulted in humans evolving in Africa rather than Asia, scientists say.
A sharply cooler and drier climate at that time, combined with upheavals of landmasses that forged the Himalayas and the Tibetan Plateau, destroyed many tropical forests in Asia. That sent surviving primates scurrying south, say paleontologist Xijun Ni of the Chinese Academy of Sciences in Beijing and his colleagues. New Chinese finds provide the first fossil evidence that the forerunners of monkeys, apes and humans, also known as anthropoids, were then largely replaced in Asia by creatures related to modern lemurs, lorises and tarsiers, the researchers conclude in the May 6 Science.
Ni’s team regards Asia as the evolutionary launching pad for primates, including anthropoids. But considerable debate and uncertainty surrounds primates’ geographic origins.
“The focal point of anthropoid evolution shifted at some point from Asia to Africa, but we didn’t understand when and why the shift occurred until now,” says paleontologist and study coauthor K. Christopher Beard of the University of Kansas in Lawrence.
But the scarcity of Asian primate fossils from that time relative to those from Africa leaves the matter unsettled. Egyptian sites in particular have yielded numerous primate fossils dating from around 37 million to 30 million years ago.
Excavations from 2008 to 2014 in southern China produced 48 teeth, some still held in jaw fragments, from six new fossil primate species, Beard says. These primates were tree dwellers and had assembled in a region located far enough south to retain forested areas. The new finds provide a rare glimpse of Asian primates that managed to weather the climate shift.
Fossil teeth of one ancient species look much like those of modern tarsiers. These tiny, bug-eyed primates now live on Southeast Asian islands. “Tarsiers are ‘living fossils’ that can trace their evolutionary history back tens of millions of years in Asia,” Beard says.
Only one Chinese fossil primate comes from an anthropoid, Ni’s group concludes. The researchers classify that animal as part of a line of Asian anthropoids previously identified from roughly 40-million-year-old tooth and jaw fragments found in Myanmar, just across China’s southwestern border (SN: 10/16/99, p. 244).
Ni’s team suspects that anthropoids evolved in Asia from earlier primates around 55 million years ago (SN: 6/29/13, p. 14). If so, anthropoids must have reached Africa before the 34-million-year-old climate shift devastated forests across Asia. Those intercontinental migrants would then have evolved into present-day monkeys, apes and humans. Investigators already knew that primates’ forest homes in Africa survived the ancient cooldown better than those in Asia.
Only one other Asian site, in Pakistan, has yielded anthropoid fossils of comparable age to the Chinese finds. The Pakistan fossils consist solely of teeth.
Asian anthropoids died out a few million years after the continent’s tropical forests began to shrink, Beard suspects.
Too few ancient Asian primate fossils have been found to say whether the southern Chinese discoveries signal a continent-wide survival of lemur and loris ancestors after 34 million years ago, says evolutionary anthropologist Blythe Williams of Duke University. The fossils also could represent an isolated population that went extinct, she says.
Lemur and loris ancestors must have lived in equatorial Africa and Madagascar by 34 million years ago, as lemurs and loris relatives do today, Williams proposes. Not enough Asian forest remained at that time to support a migration of primates discovered by Ni’s team to Africa or Madagascar, she suspects.
Williams is also skeptical of Ni and colleagues’ contention that primates originated in Asia. The oldest known primate fossils, from 56 million to 55 million years ago, come from Asia, Europe, Morocco and North America. “We cannot say where primates evolved,” Williams says.
X. Ni et al. Oligocene primates from China reveal divergence between African and Asian primate evolution. Science. Vol. 352, May 6, 2016, p. 673. doi: 10.1126/science.aaf2107.