Few groups of animals hold such special significance for us as the primates – the apes, monkeys, lemurs and more. This is the group that we are a part of. Its members are familiar and charismatic, but our evolutionary history is tangled and occasionally controversial.
Now, Polina Perelman has provided the most comprehensive view of the primate family tree to date. Her team sequenced genes from over 186 species, representing 90% of all the genera that we know of. Her tree confirms some past ideas about primate evolution and clarifies other controversies. It’s a story of island conquests, shrinking bodies, tangled branches and ancient relics.
Today, the primates’ closest living relatives are the flying lemurs, or colugos, of Southeast Asia. There are two species that both glide between trees, using flaps of skin outstretched between their legs. They can’t actually fly and they’re not really lemurs, making them the second most inaccurately named animal, after Michael Winner.
Flying lemurs aside, our next closest relatives are the treeshrews, which also live in Southeast Asia. Continuing the theme of inaccurate names, they aren’t true shrews, but they do at least (largely) live in trees.
The oldest fossil primates, such as Plesiadapis (pictured), lived around 56 million years ago, but genetic studies suggest that the group may have arisen even earlier than that. Perelman says that her study provides “strong evidence” that the first primates arose from a common ancestor around 90 million years ago. That was around the middle of the Cretaceous period, when dinosaurs like Tyrannosaurus and Triceratops were still around. It’s not clear where the primates first arose but Asia’s the best guess, given that the flying lemurs, treeshrews and many of the earliest primates are all confined there.
No sooner had the primate family tree established itself than it split into two major trunks, around 87 million years ago. The first group – the wet-nosed strepsirrhines – include lemurs, lorises and bushbabies. We belong to the other group, the dry-nosed haplorrhines, which also includes tarsiers, monkeys and other apes.
Once they had split away from the haplorrhines, the strepsirrhines themselves diverged into two major lineages around 69 million years ago. The first gave rise to the Lorisiformes, a group that includes the slow-deliberate lorises and pottos, and the spectacular leaping bushbabies. They diversified around 40 million years ago.
The second lineage gave rise to the Chiromyiformes, solely represented today by the bizarre aye-aye, and the Lemuriformes, which include all other lemurs. There are more than a hundred species and subspecies of lemur and they all live on the island of Madagascar. Every one of them descended from a common ancestor that washed up on the island’s shores around 59 million years ago.
The slow lorises have the dubious honour of being the only poisonous primates. A gland on the inside of their elbows secretes a poison that smells a bit like sweaty socks; the loris licks this gland, which gives it a toxic (and agonising) bite. The protein behind the poison is remarkably similar to the one that causes cat allergies.
The lorises as a group have another honour. Perelman found that they are the most divergent of all the primates. If you compared the genes of the seven or so species, you’d see differences that are 4 to 5 times greater than those between humans, chimps, gorillas and orangutans. For now, no one knows why.
Carl Zimmer once described the aye-aye to me as a “furry Gollum”. This unusual creature is the most ancient of all the lemurs. Its ancestors were among the first to split away from the main lemur line after it arrived on Madagascar. It hunts for grubs at night by tapping on tree trunks with its grossly distended middle finger and listening out with large ears. If it hears the right sounds, it gnaws away at the bark with rodent-like teeth and hauls the grub out with the same narrow finger, wielded like a hook.
The aye-aye aside, the remaining lemurs diverged into four main groups, starting 39 million years ago. The Lemuridae or so-called “true lemurs” were the first to emerge – today, they include the sociable and distinctive ring-tailed lemurs (left). The Indriidae were next – they include the group’s largest and most vocal member, the indri (middle), as well as the woolly lemurs and the agile, bounding sifakas. The remaining lineage split into the sportive lemurs (Lepilemuridae), and the dwarf and mouse lemurs (Cheirogaleidae; right). The latter include the smallest members of the group. Madame Berthe’s mouse lemur is the smallest of them all – it can weigh as little as 30 grams.
While the strepsirrhines were diversifying, so too were the haplorrhines. The tarsiers were the first to branch away, around 81 million years ago (more on them in the next slide). The rest of the group (the Simiformes or “simians”) split into two main lineages around 44 million years ago. These were the “flat-nosed” platyrrhines (New World monkeys) and the “narrow-nosed” catarrhines (Old World monkeys and apes).
The tarsiers have been a particularly difficult group to place. Originally, they were grouped together with the strepsirrhines to form the prosimians, from the Greek meaning “before ape”. This group – essentially all primates except monkeys and apes – has less relevance today, because we know that the tarsiers are actually haplorrhines. Perelman’s study confirms that.
These big-eyed, knobbly-fingered animals are found only in the Philippines and three Indonesia islands. But around 50 million years ago, there were tarsiers all over the Northern Hemisphere. Today’s species are but a shadow of a once diverse group, one that branched off early from other primates and has evolved alongside us ever since.
Modern platyrrhines live in Central and South America but it’s not entirely clear how their common ancestor got there. At the time, around 25 million years ago, the Panama land bridge that connected North and South American hadn’t formed, and the Atlantic Ocean was narrower. It’s possible that this ancient monkey rafted across from Africa. No matter how it got there, what happened next is clearer thanks to Perelman’s study.
After they reached South America, the platyrrhines diverged into three major families. The first to branch off were the pithecids, including the titis, the bald-faced uakaris (top left), the bearded sakis. Next came the atelids with their long, prehensile tails, including the howler (bottom right), spider and woolly monkeys.
Finally, the cebids. This group includes several species that have previously been classified in separate families; Perelman has decided to united them in one. They diverged in quick succession – first, the capuchins and squirrel monkeys (bottom left), and then, the marmosets, tamarins (top right) and the mysterious owl monkey.
The cebids are particularly interesting because as they diverged, they also became smaller. The group’s earliest members, including the night monkey and capuchins are generally larger than the later marmosets and tamarins. The smallest of the them all – the pygmy marmoset, no bigger than a hand – is also the latest to evolve.
Meanwhile, in the Old World, the catarrhines had also diverged into three major families. The cercopithecoids, including all the Asian and African monkeys, branched away 32 million years ago and started truly diversifying around 18 million years ago. The remaining catarrhines split into two groups just 20 million years ago – the hylobatids, including all the gibbons; and the hominids, including ourselves and the other great apes.
The history of the cercopithecoids is a convoluted puzzle, not least because the genetic differences within the group are lower than expected. As they evolved, it seems that many subspecies and species mated with one another to produce hybrid lineages. They turned a neat forking tree into a tangled bush.
Despite the complex history of the cercopithecoids, Perelman thinks that the family splits into two big sub-families. The Colobinae started diversifying around 12 million years ago, but they’ve given rise to a large number of African and Asian species. Most of them live in trees and eat leaves These include the colobus monkeys (far left), the langurs and leaf monkeys (left), and the aptly-named “odd-nosed monkeys” (including the bizarre proboscis monkey (right) and China’s beautiful golden snub-nosed monkey (far right)).
The second cercopithecoid subfamily – confusingly known as the Cercopithecinae – diversified at around the same time as the Colobinae. They split into two tribes. One includes some of Africa’s most beautiful species including the guenons, patas monkey, green monkeys and vervets. The other includes baboons, geladas, mangabeys, mandrills and macaques. This second group in particular has a rich history of hybridisation.
They hylobatids, or gibbons, diversified by 9 million years ago and today, there are around a dozen or so species. The largest of them – the siamang – is pictured above. These “lesser apes” have taken the primates’ fondness for trees to a whole new level. Their wrist is made up of a ball-and-socket joint, much like our shoulders or hips. That means a swinging gibbon can rotate its entire body around its wrist, giving them a unique style of movement called brachiation (video). They can zoom through treetops with a top speed of 35 miles per hour.
While the gibbons’ movements are all style and grace, their chromosomes are a chaotic mess. They’ve rearranged around 10-20 times faster than most other mammals and, as with lorises, it’s not clear why. That’s a mystery for a future study to solve.
Finally, we come to our small branch of the primate family tree – the hominids. If you follow the forking branches to us, the orangutan subfamily (Ponginae) were the first to split away around 16.5 million years ago. That branch later diverged into the two modern species of orangutan – the Bornean and Sumatran – just over one million years ago. On the other subfamily (Homininae), the gorillas were the next to branch away around 8.3 million years ago. Finally, our ancestors diverged from those of chimpanzees and bonobos around 6.6 million years ago.
The genus Homo has been around for less than 10% of the entire history of the primate order. And it has taken us far less time to put many of the other species at risk of extinction. Nearly half of all species are endangered thanks to a combination of deforestation, bushmeat hunting and illegal wildlife trade.
Reference: Perelman P, Johnson WE, Roos C, Seuánez HN, Horvath JE, et al. (2011) A Molecular Phylogeny of Living Primates. PLoS Genet 7(3): e1001342.