An international team of researchers has reconstructed the common ancestor of placental mammals—an extremely diverse group including animals ranging from rodents to whales to humans—using the world's largest dataset of both genetic and physical traits. In research to be published in the journal Science, the scientists reveal that, contradictory to a commonly held theory, placental mammals did not diversify into their present-day lineages until after the extinction event that eliminated non-avian dinosaurs, and about 70 percent of all species on Earth, some 65 million years ago. This finding, and the visualization of the placental ancestor—a small, insect-eating animal— was made with the help of a powerful cloud-based and publicly accessible database called MorphoBank. The Science article is the result of a multi-year collaborative project funded by the National Science Foundation's Assembling the Tree of Life program.HERESY! One does not DOUBT the Cretaceous Ecological Revolution! Send in the Spanish Inquisition! Oh. Wait. The 49ers shot them.
"Analysis of this massive dataset shows that placental mammals did not originate during the Mesozoic," said lead author Maureen O'Leary, an associate professor in the Department of Anatomical Sciences in the School of Medicine at Stony Brook University and a research associate at the American Museum of Natural History. "Species like rodents and primates did not share the Earth with non-avian dinosaurs but arose from a common ancestor—a small, insect-eating, scampering animal—shortly after the dinosaurs' demise."
There are two major types of data for building evolutionary trees of life: phenomic data, observational traits such as anatomy and behavior; and genomic data encoded by DNA. Some scholars have argued that integration of both is necessary for robust tree-building because examining only one type of data (genomic or phenomic) leaves out significant information. The evolutionary history of placental mammals, for example, has been interpreted in very different ways depending on the data analyzed. One leading analysis based on genomic data alone predicted that a number of placental mammal lineages existed in the Late Cretaceous and survived the Cretaceous-Paleogene (KPg) extinction. Other analyses place the start of placental mammals near this boundary, and still others set their origin after this event.
"There are over 5,100 living placental species and they exhibit enormous diversity, varying greatly in size, locomotor ability, and brain size," said Nancy Simmons, paper author and a curator in the Department of Mammalogy at the American Museum of Natural History. "Given this diversity, it's of great interest to know when and how this clade first began evolving and diversifying."
The new study combines genomic and phenomic data in a simultaneous analysis for a more complete picture of the tree of life.
"Despite the considerable contributions of DNA sequence data to the study of species relationships, phenomic data have a major role in the direct reconstruction of trees," said author Michael Novacek, senior vice president, provost for science, and a curator of paleontology at the American Museum of Natural History. "Such data include features preserved in fossils where DNA recovery may be impossible. The mammalian record is notably enriched with well-preserved fossils, and we don't want to build trees without using the direct evidence that these fossils contribute."
"Discovering the tree of life is like piecing together a crime scene—it is a story that happened in the past that you can't repeat," O'Leary said. "Just like with a crime scene, the new tools of DNA add important information, but so do other physical clues like a body or, in the scientific realm, fossils, and anatomy. Combining all the evidence produces the most informed reconstruction of a past event."
The tree of life produced in this study shows that placental mammals arose rapidly after the KPg extinction, with the original ancestor speciating 200,000-400,000 years after the event.
"This is about 36 million years later than the prediction based on purely genetic data," said Marcelo Weksler, an author and research associate in the Museum's Department of Mammalogy who is now at the Museu Nacional-UFRJ in Brazil.
The finding also contradicts a genomics-based model called the "Cretaceous-Terrestrial Revolution" that argues that the impetus for placental mammal speciation was the fragmentation of supercontinent Gondwana during the Jurassic and Cretaceous, millions of years earlier than the KPg event.
"The new tree indicates that the fragmentation of Gondwana came well before the origin of placental mammals and is an unrelated event," said John Wible, paper author and curator of mammals at the Carnegie Museum of Natural History.