The story of the rise of humanity is an intricate tapestry of events. Simple life had to begin after the early Earth was bombarded by asteroids and bathed in magma. A magnetic field had to be generated and strengthened for complex life to survive the harsh solar radiation. The non-avian dinosaurs had to perish so that mammals could take up the mantle.
Then, by the time primates evolved around 55 million years ago, the stage was set. One evolutionary lineage would survive up until the present, so that Homo sapiens sapiens – anatomically modern humans – would rule the planet. But, as a new study published in the journal Science highlights, humanity’s emergence wasn’t a sure bet.
An international team of researchers has described a “mother lode” of primate fossils, dating back to the start of the Oligocene period 34 million years ago, which they’ve unearthed in southern China. During this time the world was undergoing drastic cooling, which made much of the world inhospitable to primates. As these Asian primate fossils help to show, many of the more monkey-like primates had died out, and lemur-like primates were beginning to take over.
“At the Eocene-Oligocene boundary, because of the rearrangement of Earth's major tectonic plates, you had a rapid drop in temperature and humidity,” K. Christopher Beard, senior curator at the University of Kansas' Biodiversity Institute and co-author of the study, said in a statement. “Primates like it warm and wet, so they faced hard times around the world.”
During the Eocene-Oligocene transition, some ape-like primates migrated from Asia to Africa, whereas the rest died out, and were overtaken by lemur-like animals. The main stage of primate evolution took place in Africa, not Asia, thereafter. Ni et al./Science
Primates were wiped out in both North America and Europe, but a branch survived in Asia. Some of these monkey-like primate populations made it to the more habitable, warmer African continent, where they began to diversify into new species. As we know, humanity emerged out of Africa, not Asia.
If this period of global cooling hadn’t occurred, and the primate populations of Asia weren't so badly affected by it, primate evolution would have likely continued at a remarkable pace. Therefore, there’s a chance that in this alternate timeline another advanced primate would be ruling the world – not us.
Although there have been previous hints that primates had their own isolated populations in Oligocene-age Asia, this treasure trove of fossils allowed the team to paint a detailed picture of what life may have been like back then for these troubled creatures. It appears that, due to the global cooling, the few surviving ancient primates were huddled together in the south of China, one of the warmest parts of the continent at the time.
The left lower jaw of Yunnanadapis folivorusm, another of the six new species found at the site. IVPP, Chinese Academy of Sciences
Based on their skeletal features, these primates were tropical tree-dwellers. One of the species, Oligotarsius rarus, was remarkably similar to the modern tarsier, which is today found only on a handful of Philippine and Indonesian islands. The teeth of these ancient critters were almost identical to their modern equivalents, revealing that tarsiers today are “living fossils,” with the same diet and behaviors as they had tens of millions of years ago.
In any case, this study confirms that primates are incredibly sensitive to rapid climate change, and that’s changing more rapidly today than ever before. It’s not just wild primates that are in trouble: Humanity itself is being pushed out of environments it once used to dominate.