Identity of placental mammal ancestor reconstructed

Have you ever wondered what the common ancestor of all placental mammals looked like? A recent study shows that we are descendants of a rat-like mammal that weighed no more than half a pound, displayed a long, furry tail and dined mostly on insects.

After six long years of investigation, researchers published a study in Science on Feb. 8 explaining their findings and a reconstruction of what seems to be the most probable ancestor of all placental mammals. This group is the most common and diverse of all mammals, and is distinguished by the “placenta,” an organ through which maternal blood comes in close contact with the fetus to provide nutrition.

The team of researchers used the publicly accessible database MorphoBank to conduct their search into the common ancestor’s identity. Not only is MorphoBank the largest record of extinct and living mammals, but it is able to handle very large amounts of possible traits for the mammal being searched.

The database has more than 12,000 images that correspond to more than 4,500 traits. It contains information about teeth and bones, the presence or absence of wings, the type of hair and even brain structures. One of the biggest advantages of this database is that it contains about ten times more information than databases used in previous studies.

The system is able to compute and assess each mammal on the basis of numerous traits. Those traits could come from genetic, anatomic and paleontological data; behaviors and molecular information were also taken into account.

“It is better to use all the data... to make sure that the results are robust,” Maureen O’Leary, principal author of the study and an associate professor of anatomical sciences at Stony Brook University’s School of Medicine, wrote in an email to The News-Letter.

This study used empirical traits of 86 placental mammal species, 40 of which were fossil species. O’Leary, also a research associate at the American Museum of Natural History, explained the benefits of utilizing soft tissues samples that the museum stores as an additional method to find the common ancestor.

The illustration that O’Leary and her colleagues were able to develop represents a conglomeration of all their data.

“We entered information in a spreadsheet for DNA and all of the anatomy, and then we ran out an algorithm, and once we did that, that introduced an evolutionary tree with the arrangements of all the species on it,” O’Leary wrote.

Using the tree, lineages could be traced back to the common ancestor of all placental mammals. The collaborators found that, contrary to previous theories, placental mammals arose after the extinction of dinosaurs 65 million years ago. From there, the researchers proceeded to form a visual representation of their findings.

“It was important for us to draw the picture so we could actually see what it looked like,” O’Leary wrote.

In the constructed illustration, it is apparent that the common ancestor had many interesting characteristics including a two-horned uterus, a brain with a convoluted cerebral cortex and, of course, a placenta just like us.