A groundbreaking six-year research collaboration has produced the most complete picture yet of the evolution of placental mammals, the group that includes humans. Placental mammals are the largest branch of the mammalian family tree, with more than 5,100 living species. Researchers from Carnegie Museum of Natural History are among the team of 23 that took part in this extensive interdisciplinary effort that utilizes molecular (DNA) and morphological (anatomy) data on an extraordinary scale. By combining these two types of data scientists reconstructed, to an unprecedented level of detail, the family tree of placental mammals. This study explored thousands of characteristics of the anatomy of both living and extinct placental mammals.
This new project produced a more complete picture of mammalian history and provides a huge dataset that will become the starting point of research for a number of scientific questions, including those of vital importance today: how mammals may have survived climate change in the past and what may that mean for our future. The paper appears today in the journal Science.
The collaboration is part of the Assembling the Tree of Life (ATOL) project funded by the National Science Foundation.
Joining forces, joining research
Today's article reveals the final results of the six-year ATOL project. The study began with two teams organizing data from two distinct approaches to evolutionary research: molecular data (DNA), and morphological data (anatomical features).
"In the field of mammal research, there had been a big divide between people working with DNA and others working on morphology," explains John Wible, PhD, Curator of Mammals at Carnegie Museum of Natural History and co-author on the paper. "They just weren't working with each other until now."
The molecular team collected DNA sequences of living animals and the morphology team examined the anatomy of both living and extinct mammals. The molecular team only sampled living mammals, because genetic material can't be extracted from fossils older than 30,000 years,. Thus, to include fossils, morphological information was essential. Researchers in morphology deal with individual physical features, from bone length to types of teeth to the presence of stripes in the fur; each one of these features is termed a 'character.' By collecting as many characters as possible and comparing their variation among dozens of specimens, relationships between species can be tested and broader patterns emerge.
The ATOL project became a morphological powerhouse. Generally, a group of 500 characters is considered to be a large dataset. The morphology researchers on the ATOL project generated an unprecedented 4,500 characters. Once both DNA and morphological datasets were produced, the resulting combined matrix provided an unprecedented amount of information for each of the 83 mammals included in the study.
"It's not that we hadn't combined morphology with DNA before." clarifies co-author Michelle Spaulding, PhD, the Rea Post-doctoral Fellow at Carnegie Museum of Natural History. "This time, we ratcheted up the amount of morphological detail phenomenally, providing a larger anatomical base for the study as compared with DNA than is typical.."
With the new Tree of Life matrix, researchers now have greater context for the fragmentary fossils they have in hand—often scant evidence such as a few teeth or a skull fragment—potentially shedding light on little-known species that have yet to find a solid home in the evolutionary tree.