We humans —along with apes, monkeys, and lemurs— are primates, and exploring primate evolution helps us understand where we came from. Indigenous to Madagascar, lemurs form a unique branch of the primate family tree. But there is one important obstacle to understanding where lemurs came from and how they evolved– there are no ancient fossil lemurs. This void makes it difficult to track the stages of evolution often observed in transitional fossil forms. Making matters more confusing, but fascinating, there are remains of lemurs extinct around 2,000 years old. Those lemurs were radically different from the ones alive today. Many of the extinct lemurs were giants, with some weighing as much as gorillas! Placing them in the tree with living species has been difficult because of their unique features. To understand the factors that may place species at risk of extinction, it is important to compare the traits the extinct species had to those of living species. For my Ph.D. research, the first step was to do just that. With my committee member and collaborator, Stony Brook University Professor Dr. Liliana M. Dávalos, I studied the evolutionary history of lemurs in detail with support from the National Science Foundation. The findings are out this week in the journal Systematic Biology. For the first time since the early 1990s, I combined data from living and extinct species with cutting-edge techniques to place all lemurs in a single tree, providing the most complete tree of lemur evolution yet.
Surprisingly, the new phylogeny finds some of the subfossil species were unique branches with no living close relatives. The giant, extinct koala lemur (Megaladapis) had short limbs and long finger and toe bones, which may have allowed it to cling to branches, much as living koalas do. Megaladapis also had an extremely long snout with no upper front teeth, and may have even had long dexterous lips for manipulating food. The koala lemur was thought to be closely related to either of two living groups: the sportive lemurs (Lepilemur), or ‘true’ lemurs, like the ring-tailed lemur (Lemuridae). Because of the unique features with few similarities to living species, placing them in the lemur tree based on skeletal traits has been difficult. Fragments of ancient DNA had suggested a close link between koala and true lemurs. By combining genetic and anatomical data, the new tree finds the koala lemur was a completely extinct, unique branch of the lemur tree.
Other fossil groups are more closely related to living species, but the fossil groups were giant compared to living species. While the largest living lemur is about 22 pounds, the largest and closely related extinct lemurs may have weighed over 300 pounds! Large body size is one important component of extinction risk, and so by understanding the evolution of body size, we can better understand the factors threatening lemurs today. The implications for the future of lemurs are substantial; 95% of living lemurs are threatened with extinction. With all living and extinct lemurs together in the same tree, we can begin to understand how evolution and extinction have worked in the past, and project risks into the future.
Another way the study helps us understand primate evolution is by illuminating how geography has contributed to splitting off species. Madagascar has been an island separate from all other landmasses for at least 90 million years, long before primates first appeared. As in previous analyses of molecular clocks, the new study suggest lemurs first evolved ~50-60 million years ago. This means the ancestor of lemurs likely arrived on Madagascar by dispersal – perhaps washed from Africa to Madagascar floating on vegetation, or with some small stepping-stone islands to help along the way. A surprising new twist arises from the new lemur family tree: could lemurs have dispersed from Madagascar back to Africa? In some analyses, there was evidence a fossil primate found on Africa 34 million years ago (Plesiopithecus) was most closely related to the aye-aye, the first lemur to diverge from other species. If confirmed, this evolutionary pattern means the aye-aye and the ancestor of all other lemurs dispersed from Africa to Madagascar separately, or the ancestor of all lemurs arrived on Madagascar and this unique branch dispersed back to Africa. This finding warrants an intense focus on the similarities between the aye-aye and the African fossil Plesiopithecus to determine whether they are the result of common ancestry, or if this link is an artifact of similar ecological pressures leading to similar traits.
With this new evolutionary tree, we have a much better understanding of the evolutionary dynamics of primates and how evolution shaped the primates of Madagascar over the last 50 – 60 million years. We can now compare the factors associated with the extinction of all subfossil lemurs, and predict how living lemurs may fare in the future. This information is crucial if we hope to prevent further extinctions of these distant human relatives.
Check out the Huffington Post's piece on this article
And Forbes too!
Illustrations copyright S. Nash, used with permission