Placental mammals evolved from small bug eater

STONY BROOK (US) — Researchers have reconstructed the common ancestor of placental mammals and describe it as a small insect-eating animal.

The study, published in the journal Science, suggests placental mammals—an extremely diverse group including animals ranging from rodents to whales to humans—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. The discovery contradicts a commonly held theory about the rise of placental mammals.

The findings, and the visualization of the placental ancestor, was made with the help of a powerful cloud-based and publicly accessible database called MorphoBank.

This shrew-sized Cretaceous-age animal, Ukhaatherium nessovi, which was uncovered in 1994 in the Gobi Desert by the Mongolian Academy and the American Museum of Natural History, is one of the many mammals used in the recent tree-of-life study. (Credit: Stony Brook University)


“Analysis of this massive dataset shows that placental mammals did not originate during the Mesozoic,” says lead author Maureen O’Leary, an associate professor 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.”

Mapping traits

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 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,” says Nancy Simmons, 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.”

As part of the study, researchers recorded phenomic traits for 86 placental mammal species, of which 40 were fossil species. The resulting dataset has more than 4,500 traits detailing characteristics such as the presence or absence of wings, teeth, and certain bones, type of hair cover, and structures found in the brain, as well as over 12,000 supporting images, all publicly available online.

The dataset is an order of magnitude (10 times) larger than what has previously been used for studies of mammal relationships.

The team reconstructed the anatomy of the placental common ancestor by mapping traits most strongly supported by the combined phenomic and genomic data and comparing the features in placental mammals with those seen in their closest relatives.

This method, known as optimization, allowed the researchers to determine what features first appeared in the common ancestor of placental mammals and also what traits were retained unchanged from more distant ancestors.

The researchers conclude that the common ancestor had features such as a two-horned uterus, a brain with a convoluted cerebral cortex, and a placenta in which the maternal blood came in close contact with the membranes surrounding the fetus, as in humans.

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,” says Marcelo Weksler, research associate at the American Museum of Natural History, who is now at the Museu Nacional-UFRJ in Brazil.

Other conclusions

In addition, the study reveals that a branch of the placental mammal tree called Afrotheria (because these animals—which range from elephants to aardvarks—live in Africa today) did not originate on that continent but rather in the Americas.

“Determining how these animals first made it to Africa is now an important research question,” says study author Fernando Perini, a former postdoctoral fellow at the American Museum of Natural History who is now a professor at the Minas Gerais Federal University in Brazil.

“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 says. “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 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,” says John Wible, curator of mammals at the Carnegie Museum of Natural History.

Researchers associated with Carnegie Museum of Natural History, University of Chicago, Florida Museum of Natural History, University of Louisville, University of Tennessee at Chattanooga, Western University of Health Sciences, Peabody Museum of Natural History, Yale University, and the Institute of Vertebrate Paleontology and Paleoanthropology (China), also participated in the study.

The National Science Foundation, Stony Brook University, the American Museum of Natural History, and the National Oceanic and Atmospheric Administration funded the project.

Source: Stony Brook University