Furry insect-eater was earliest humans and most mammals' ancestor
Washington, February 8 : Scientists have been able to predict the appearance of the most recent common ancestor of all placental mammals - a small furry long-tailed insect-eating creature.
The groundbreaking six-year research collaboration also 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 collaboration is part of the Assembling the Tree of Life (ATOL) project funded by the National Science Foundation.
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.
Thanks to the incredible amount of anatomical information collected, the researchers were able to reconstruct the animal that gave rise to every placental mammal following the extinction of the dinosaurs.
"We looked at all aspects of mammalian anatomy, from the skull and skeleton, to the teeth, to internal organs, to muscles, and even fur patterns. Using the new family tree of mammals in tandem with this anatomical data, we were able to reconstruct what this common placental ancestor may have looked like," said Wible.
The scientists were able to work with an artist to approximate the appearance of this ancestor. While only hypothetical, the illustrated concept for this ancestor-from body size to fur type to number of teeth-could not have been achieved prior to the Herculean task of developing the matrix.
"We focused our study on the time around the Cretaceous-Tertiary (KT) boundary, 65 million years ago," states Spaulding.
"Molecular and morphological based studies differ on the age when placentals first appeared. Molecular studies place the origin in the mid-Cretaceous, when dinosaurs still dominated. On the other hand, morphological studies have routinely found no evidence of any placental fossils in this time period, and instead place the placental origin after the mass extinction at the close of the Cretaceous that ended dinosaur dominance," he added.
One major goal of this project was to address this controversy and results found that placental mammals appeared after the KT boundary, implying that the mass extinction was a critical event in mammalian evolutionary history.
The results appeared in the journal Science.