Dead monkeys do tell tales

A study, published in the journal Proceedings of the National Academies of Sciences reveals that the species Xenothrix mcgregori was a close relative of South American Titi monkeys, from the family Callicebinae. Earlier research had suggested the primate was related to other extant families, or possibly represented a unique phylum.  

The analysis – conducted by researchers from the UK’s Natural History Museum, the Zoological Society of London and the American Museum of Natural History – is the latest in a growing catalogue of work that uses scraps of DNA extracted from long-buried bones to illuminate evolutionary relationships. 

Whereas fossils encased in the Siberian permafrost can yield ancient DNA tens of thousands of years old, those in the tropics rarely produce even incomplete genetic sequences. 

“Temperature and humidity destroy DNA very quickly,” explains molecular evolutionary biologist Ian Barnes from the Natural History Museum.

But he and his colleagues, got lucky. They painstakingly extracted DNA from a Xenothrix leg bone that had been sitting in the American Museum of Natural History in New York ever since it was collected from a Jamaican cave in 1920.

By comparing its mitochondrial and genomic DNA sequences to sequences from other Central and South American primates – the so-called New World monkeys – the team was able to construct a family tree showing how they are all related. 

Xenothrix, it turns out, is closely related to Titi monkeys. Most Titi species only differ from each other in size and the colour of their fur – variations brought about over time as rivers prevented adjacent populations from mating, thus inducing speciation. 

Judging from anatomy alone, the Jamaican primate’s close relationship with the Titis seemed unlikely. Its molar teeth had suggested a closer link to other monkey families.  

The likely explanation for its unusual anatomy: Xenothrix’s home. 

Islands are hotbeds of biological diversity. Unlike their mainland counterparts, island dwelling animals can undergo rapid evolutionary change that catapults them into extreme forms vastly different from their ancestors or mainland relatives. 

Dwarf elephants and pygmy hippopotamuses, along with giant tortoises and elephant birds are the products of this sort of run-away evolutionary process. The diminutive Homo floresiensis is thought to be a case of island dwarfism in a line of early humans living in current-day Indonesia.  

“This new understanding of the evolutionary history of Xenothrix shows that evolution can take unexpected paths when animals colonise islands and are exposed to new environments,” says co-author Samuel Turvey.

The monkey split from the Titi monkeys about 11 million years ago, but primates have existed on Jamaica for even longer than that, according to the analysis. This suggests that they made the watery journey to the Caribbean more than once. 

It also means that other monkeys in the island region didn’t all evolve from a single ancestor. 

If that’s the case, even ancient DNA might struggle to tell the complex story of the evolutionary origins of the primates of the Caribbean. 

“I don’t think that this is really the last word on what’s happening here,” says primate palaeoanthropologist Varsha Pilbrow from the University of Melbourne, who was not involved in the study. “What’s happened in the Caribbean is really a dog’s breakfast.”

With more ancient DNA in his hands, Barnes hopes to get a greater understanding of what genetic changes may have led to Xenothrix’s strange anatomy, and whether its traits helped it adapt to its island home. 

“We might be able to identify what genes are responsible for the changes that we’re actually seeing, and from there, actually start to think about a better understanding of how the evolutionary process happened,” he says.