How gibbons became lords of the treetops
The diversity of gibbons has long fascinated zoologists. What was their story? Elizabeth Finkel investigates.
Dawn in the forests of Southeast Asia is marked by a gibbon song contest – each mating pair sings a unique duet to mark their tree-top patch. And that’s just one of the remarkable adaptations gibbons have made to life in the canopy. With their flexible joints, hook-shaped hands and oversized arms, gibbons fly through the trees at speeds of 56 km/h and walk tall on the limbs of high branches with the grace of acrobats.
How did they adapt so exquisitely to the high life? Possibly because their DNA is riddled with unstable stretches of DNA that have churned up their genetic code, say Lucia Carbone at Oregon Health & Science University and her colleagues, who published their findings this September in Nature.
Gibbons occupy a unique spot in the primate tree. Their ancestor (and ours) branched away from old-world monkeys such as macaques and baboons, to give rise to the apes. A characteristic of apes is that they are highly adapted to swinging from the trees with their arms, in a motion known as brachiation.
Gibbons were the first swingers to appear – and they remain virtuosos.
“No species are as adapted to brachial locomotion so well as gibbons. This can be seen by the sheer speed they can move through the trees with a swinging motion, which is frankly quite incredible,” says Krishna Veeramah, who studies primate genomes at Stony Brook University, New York.
The great apes emerged over the 17 million years after the gibbons appeared: chimps, gorillas, orang-utans and humans. But while each great ape is represented by one or two species, gibbons are far more diverse. There are 19 species divided between four groups, or genera.
This diversity of gibbons has long fascinated zoologists. What was their story?
Carbone and her colleagues had two clues. The first related to geography. Isolated populations evolve more rapidly because new mutations have a chance of spreading rather than being diluted in the common gene pool. And five million years ago, rising sea levels in South East Asia cut gibbon populations off from each other.
The second clue was in the gibbons’ chromosomes. Chimps, gorillas and orang-utans all have a stable 24 sets, and humans have 23 (because two fused together). But among gibbons, chromosome sets are all over the place, ranging in number from 38 to 52.
To explain the mess, Carbone looked deeper into the genetic code of gibbons. Her initial hope had been that, by comparing the DNA codes of the four different genera, they would be able to determine the order in which the different groups evolved.
Gibbons swing tirelessly through the trees. Orang-utans,
by contrast, can only swing for two minutes.
Carbone likens the genome to a landscape. Slow erosion usually takes place, making it possible to trace how one genome evolved into another by looking at the mutations they share. But that proved impossible when looking at the genomes of gibbons. “Their divergence occurred too rapidly,” she says. The best Carbone’s team could do was estimate that all the gibbons evolved from a common ancestor during a five million year burst. “In evolutionary terms that’s almost instantaneous,” says Carbone. The genetic analysis placed the burst at the period when sea levels began rising in Southeast Asia.
But the scrambled chromosomes played their part too. Rather than slowly eroding, “the gibbon’s genome appeared to have experienced earthquakes”, says Carbone.
Peering at the genome data, Carbone stumbled across the smoking gun that could explain the genome quakes. She found the mischievous DNA snippets, called LAVA, that have a tendency to multiply and jump around, scrambling the DNA code as they go. “We found thousands of them that showed signs of being recently active,” says Carbone. Humans also carry such “jumping genes”, but they are sluggish by comparison. Although they have been linked to autism and cancers they haven’t succeeded in scrambling our chromosomes.
But earthquakes sometimes uncover treasures. In the case of gibbons, their hyperactive LAVA may be the key to the animals’ extreme diversity and their remarkable adaptations.
Two bits of circumstantial evidence implicate the LAVA elements in the rapid evolution of gibbons. One is that they were found lurking nearby and disturbing genes that stabilise chromosomes – which in itself could lead to a host of new genetic variations. But in addition, other LAVA elements were also found near genes that had clearly evolved rapidly in the gibbons. Some were implicated in cartilage development and could help explain the ability of gibbons to swing tirelessly through the trees. Orang-utans, by contrast, can only swing for two minutes.
“LAVA has its signature in all the right places to be the smoking gun,” agrees Michael O’Neill, a genomics researcher at the University of Connecticut. But he warns, “correlation is not causation”.
Gibbons aren’t the only creatures in which rapid chromosome changes seem to have been triggered by "jumping genes". Wallabies, grasshoppers and the horse family (including zebras and donkeys) tell a similar story of rapid evolution, multiple species with scrambled chromosome sets, and the evidence of "jumping genes" at work.
“Nobody understands how this rapid evolution happens, while other species like camels and llama are stable for millions of years,” says Rachel O’Neill, also at the University of Connecticut. “Across the board, we see 'jumping genes' associated with rapid chromosome changes. That makes the smoking gun a lot hotter,” adds Michael O’Neill.