Descended from Rabbits? New primate family tree tells all
Richard Dawkins, in his book, The Ancestor’s Tale, takes readers on an imaginary pilgrimage back through 4 billion years of evolution. The path he follows is one of a reverse chronology beginning with humans. Along the way humans are joined at “Rendezvous Points” by creatures with which we share a common ancestor – “concestors”, he calls them. We are, for example, joined at Rendezvous 1 (between 5 and 7 million years ago) by chimpanzees. It is at this juncture that chimps and bonobos figuratively went left, while humans went right, leaving behind some intermediary concestor that probably looked and acted a bit like both of us.
Continuing Dawkins’ guided walk, humans are joined later (or earlier?) by orangutans (our first step out of Africa), gibbons, old world monkeys, new world monkeys, tarsiers, and then, at Rendezvous 8 (around 63 million years ago) by a group that include lemurs, bushbabies, and their kin. The ancestor we share with these creatures is considered to be our oldest known primate progenitor.
And it’s at this point that that a new story begins.
Last week in PLOS Genetics, a multinational team of scientists from the US, Brazil, France and Germany published the first comprehensive primate family tree. Using DNA samples representing 90% of all living primates (about 186 species), the authors conducted a phylogenetic analysis to create an indisputable family tree based purely on genetic data.
Like zooming down into a Google map, this new “high-resolution” view enables scientists to look deep into the finer details of the relationships among the different primate species and will provide better insight into the origin and evolution of primates.
Much of what the tree reveals has been known by scientist – or was at least strongly hypothesized based on current knowledge. However, the authors point out that their added data clarifies numerous taxonomic controversies and provides new information specific to human evolution.
According to the authors, primate’s closest-related mammalian order is Dermoptera, the flying lemurs. (Interestingly, this order is rooted in Lagamorpha, so in essence, we all come from rabbits.) About 87 million years ago, Dermoptera split into the two primate lineages – the strepsirrhines (lemurs, lorises and their kin) and haplorrhines (tarsiers and true simians). From this second group emerged all of the old world monkeys, new world monkeys and apes (including us) that we recognize today.
Focusing on our own immediate ancestors and closest kin, the study confirms chimpanzees as being the human’s nearest relative. This is not surprising as the relationship has been generally undisputed. However, looking more closely at Old World Monkeys as a whole, an intriguing phenomenon emerges – evidence of an evolutionary slow-down that permeates the entire group. According to Jill Pecon-Slattery of the Laboratory of Genomic Diversity at the US National Cancer Institute in Maryland, as quoted in The Independent:
“One of the most interesting findings is that humans are not evolving faster than other primate lineages, but in fact are among the slowest. This is not a new idea, as it is called the ‘hominoid slow-down’, but what is new is that it is not unique to humans.”
The importance of understanding the genetic relationships among primates extends beyond just academics. For instance, as genetically close as we are to chimpanzees, their susceptibility to certain diseases widely diverges from our own in several well-documented cases. AIDS and malaria, for example, kill millions of humans every year, but chimpanzees appear to be largely immune to both. What differences in our genetic history account for this? A close look at our shared inheritance and lineage-specific immune responses may help reveal answers to questions like that.
Finally, in an interesting footnote, primate behaviourist Eric Johnson, on his blog, noted that the study appears to show that the relationship between humans and bonobos (Pan paniscus) is closer than that of humans and chimpanzees (Pan troglodytes). He pulled this quoted from the study:
“Based on 54 gene regions, Homo-Pan genetic distance range from 6.92 to 7.90×10−3 substitutions/site (P. paniscus and P. troglodytes, respectively), which is less than previous estimates based on large scale sequencing of specific regions such as chromosome 7.”
His post attracted a lot of attention however, after corresponding with the authors, they explained that:
Common chimpanzee diversification into subspecies (two of which are included in our study) may be correlated with increased rates of genome evolution, thus making them appear more distantly related to humans than bonobos. There is a slight hint of that speed-up in the estimates of substitution rate for P. verus, but the overlapping HPD values across Pan make it only an intriguing speculation.
Kudos to Johnson for getting a clarification on that, but I must admit I’m disappointed. I’d much rather think myself more closely related to those peace-loving and delightfully sexual bonobos than their more aggressive chimpanzee cousins. But barring that, I will happily settle for being descended from rabbits.
Citation: Perelman P, Johnson WE, Roos C, Seuánez HN, Horvath JE, et al. (2011) A Molecular Phylogeny of Living Primates. PLoS Genet 7(3): e1001342. doi:10.1371/journal.pgen.1001342
Citation: Barreiro LB, Marioni JC, Blekhman R, Stephens M, Gilad Y (2010) Functional Comparison of Innate Immune Signaling Pathways in Primates. PLoS Genet 6(12): e1001249. doi:10.1371/journal.pgen.1001249