Improbable voyages- did an ancient ocean crossing give rise to African primates- and humans?
Humans evolved in southern Africa around 200,000- 350,00,000 years ago, then expanded out to the rest of the world around 100,000 years ago. Several lines of evidence point to an African origin for Homo sapiens. Human diversity is highest there, both in terms of genes and languages, suggesting a long history on the African continent. There’s also an extensive fossil record of primitive humans. What’s more, our closest relatives, chimps and gorillas, live in Africa, along with a diverse fauna of baboons and monkeys like vervets and colobus. Humans seem to be part of a radiation of higher primates, apes and monkeys, native to Africa. But how did primates even get there in the first place?
Apes and monkeys seem to originate in Africa, but primates themselves didn’t evolve there. Monkeys’ closest relatives, tarsiers, are Asian. And the closest relatives of primates, flying lemurs, tree shrews, rodents and rabbits, are all either restricted to Asia, or in the case of rodents and rabbits, evolved there. Modern biogeographic patterns seem to hint at an Asian origin for primates.
Fossils, meanwhile, provide somewhat conflicting evidence, but the one thing they make clear is that primates aren’t African. The earliest primate relative, Purgatorius, lived just 65 million years ago, just a million years after the dinosaurs disappeared. It comes from Montana. Meanwhile, the Plesiadapiformes- an extinct group related to primates or perhaps representing primitive primates- are widespread in Asia, North America, and Europe- but not Africa.
The oldest true primates are likewise non-African. The lemur relative Darwinius comes from Europe. Teilhardina, related to tarsiers, monkeys and apes, lived around 55 million years ago. It’s found in Asia, North America, and Europe but not in Africa. Meanwhile, the fossil record shows that primates arrived millions of years later in Africa. Lemur relatives appear in Africa 55 million years ago, and monkey-like forms around 40 million years ago.
Africa at this time wasn’t connected to Asia, or any other continents. When Africa split off from South America around 100 million years ago, it became an island continent, and sometimes an archipelago, surrounded by deep oceans. That means the ancestors of lemurs and monkeys had to cross water to get there.
The idea of oceanic dispersal is central to the theory of evolution. Studying the Galapagos Islands, Darwin saw a low diversity fauna, with a handful of tortoises, lizards, snakes, and one small mammal, a rice-rat. Farther out to sea, on islands like Tahiti, were only a few small lizards. Darwin reasoned that if God created animals, He could have put anything anywhere. But these patterns only made sense if, over time, animals crossed water to colonize islands. Rafts of debris and vegetation, or floating islands- stands of trees swept out to sea- could carry lizards, snakes, and rice-rats the 1,000 km from South America to Galapagos. Tortoises can survive long periods adrift without food or water, and probably bobbed along until they hit the Galapagos. Farther out at sea, only a few tiny lizards survived the crossing.
The odds are against successful crossings. Many animals on rafts swept offshore succumb to thirst, starvation, or sharks long before they hit an island, most just never make landfall at all, and are lost at sea. That’s why oceanic islands, especially distant ones, have few species. So oceanic dispersal has been treated as a novelty in evolution- a curious thing that happens in little places like Galapagos, but not relevant to the evolution of major animal groups, or large continents. In fact, it appears critical to evolution.
Continental drift was once believed to explain how animals moved between distant continents. However, it’s now become clear that the continents split up too long ago to explain most modern distributions. The supercontinent Pangaea started to break up in the early Jurassic, 200 million years ago, and breakup was completed around 100 million years ago, before modern mammals, birds, and reptiles evolved and became widespread. Meanwhile, Africa didn’t connect to Asia until around 20 million years ago, and South America was isolated until 3 million years ago. That means rafting is likely to have played a role in the distribution of primates- and other animals.
Several rafting events are well-established for primates. Madagascar has a diverse fauna of lemurs- mouse lemurs, aye-ayes, indris, ringtails, and until recently, giant, gorilla-like lemurs. They arrived 50 million years ago, from Africa. Since Madagascar has been isolated since the time of the dinosaurs, lemurs apparently crossed the 400-km-wide Mozambique Channel by rafting on vegetation. Even more incredible is the existence of monkeys in South America- howlers, spider monkeys, marmosets. They arrived 35 million years ago- again, from Africa. To do so, they had to cross the Atlantic, which was narrower then, but still a wide ocean. The right combination of conditions- a large raft of vegetation, a viable population, the right currents and winds, a well-timed landfall- would be necessary to ferry monkeys across the Atlantic. And from there, monkeys rafted yet again, to the Caribbean, then to North America.
But before any of this could happen, rafting events had to populate Africa with primates. One brought the ancestors of lemurs. The other brought the ancestors of monkeys, apes- and us.
Rafting events aren’t just a curiosity, or a peculiarity of islands. They explain the origins of major groups, not only primates, and the fauna and flora of continents. Rafting explains how rodents colonized Africa, then South America. The Afrotheria, which includes elephants, aardvarks and elephant shrews, likewise must have crossed water to get to Africa. Marsupials apparently rafted from North America to South America, then Antarctica, and finally Australia. South America’s extinct hoofed mammals, Toxodon and kin, probably rafted from North America as well; the ancestors of sloths probably rafted into South America. Other oceanic crossings include mice to Australia, and tenrecs, civets and hippos to Madagascar. Whale ancestors swam to India, then an island continent. Many of these crossings probably happened in the wake of the end-Cretaceous asteroid impact, when a lack of predators and competitors made it easy for colonists to become established.
If we look at lizards, snakes, various groups of frogs, it’s the same thing. Geckos crossed oceans repeatedly. Iguanas jumped from North America into South America, Madagascar and even Fiji, over 10,000 km away. Burrowing worm-lizards crossed the Atlantic three times. And before then, we had dinosaurs crossing oceans- duckbills and titanosaurs.
Oceanic crossings, then, aren’t an evolutionary subplot, they’re central to the story. They explain the evolution of elephants, kangaroos, whales, and ourselves.
These are obviously low-probability, literally once-in-a-million years events. But given tens of millions of years, once-in-a-million-years events will happen, on average, every million years. Darwin saw evolution as being driven by the action of everyday events- competition, predation, mate choice- over vast spans of time. Doubtless much of evolution is. But over millions or billions of years, rare, low-probability, high impact events- “ black swan” events- become increasingly important. Some are immensely destructive- asteroid impacts, massive volcanic eruptions, ice ages. But others are positive- genome duplications, complex adaptations like human language, and unlikely rafting events. Given time, improbable events become not just probable, but near certainties.
The role oceanic rafting plays in our history shows just how much evolution comes down to chance. Had anything gone differently- the weather was bad, the seas were rough, the raft washed up on a reef, or a desert island, there were no males on the raft, or hungry predators on the beach, too many competing species- the rafting event would fail. Our ancestors beat odds that make Powerball lotteries seem like a safe bet. What might have happened if anything had gone differently? If any number of failed crossings had succeeded- or ours failed- the evolution of life on Earth might look very, very different than it does, and we wouldn’t be here to wonder about it.
Originally published at https://www.nicklongrich.com on March 12, 2021.
