The placenta is the lifeline for mammalian reproduction and a complex organ that provides clues about natural selection and evolution.

Through phylogenetic analysis of molecular and anatomical data, researchers from Wayne State University (WSU) and the Perinatology Research Branch (National Institute of Child Health and Human Development/National Institutes of Health) have presented evidence describing the evolutionary history of the placenta of eutherian mammals—the group that includes all mammal species except marsupials and the egg laying monotremes (for example, the duck billed platypus). The authors conclude that the human placenta—although different from the ancestral type—resembles the original eutherian placenta in many aspects.

Early placental structures (existence of particular membranes) can be inferred from the fossil evidence of Ichthyosaurus, more than 170 million years ago. A variety of placenta-like structures are also found in squamate or scaly reptiles. The placenta allows live births, as opposed to hatching eggs, and gives mammals their place on the evolutionary tree.

In contrast to most theories, the PNAS authors say the disc-shaped, hemochorial placenta of many primates, including humans, existed throughout the eutherian lineage from the last common ancestor of placental mammals to the emergence of humans. These findings are published in the February 28 issue of the Proceedings of the National Academy of Sciences (PNAS) and suggest that evolutionary pressures shaped mammal placentas to be an organ that must meet the demands of both the mother and the developing fetus during pregnancy. (Early edition now available online at http://www.pnas.org/.)

Primary author Derek Wildman, Ph.D., assistant professor in WSU’s Center for Molecular Medicine and Genetics and Department of Obstetrics and Gynecology and member of the Perinatology Research Branch, NICHD/NIH, says, in the past many biologists assumed that the placental structures of humans and other “higher” primates were advanced among mammals and other classes. This newly published evidence, however, reconstructs the ancestral eutherian placenta to be similar to the human placenta in terms of its disc-like shape and its ability to invade deeply into the maternal uterus (this maternofetal interface is called hemochorial in technical language). However, unlike the placentas of ancestral mammals, human placentas transfer nutrients, gases and other materials through a tree-like branching system of villi.

The placenta of marsupials (kangaroos and koalas, for example) is different than that of eutherians, because the presence of the placenta is very short-lived and contributes more to embryonic than to fetal nourishment. Differences and variations also exist within the eutherian group. For example, the placenta of rodents differs from primates in that rodent fetuses have a much shorter gestation and may use maternal resources at a more rapid rate. The evolution of the primate placenta required some changes necessary to sustain a longer pregnancy without depleting maternal resources and energy, the authors said.

The report, “Evolution of the Mammalian Placenta Revealed by Phylogenetic Analysis,” was contributed to the PNAS by Morris Goodman, Ph.D., WSU distinguished professor, who is a member of the National Academy of Sciences. It can be viewed online at: http://www.pnas.org/. Co-authors are: Derek Wildman (WSU Center for Molecular Medicine and Genetics; Department of Obstetrics and Gynecology; Perinatology Research Branch, NICHD, National Institutes of Health); Caoyi Chen (WSU Center for Molecular Medicine and Genetics); Offer Erez (Perinatology Research Branch, NICHD, National Institutes of Health; WSU Department of Obstetrics and Gynecology); Lawrence I. Grossman (WSU Center for Molecular Medicine and Genetics); Morris Goodman (WSU Department of Anatomy and Cell Biology; Center for Molecular Medicine and Genetics); and Roberto Romero (Perinatology Research Branch, NICHD, National Institutes of Health; WSU Department of Obstetrics and Gynecology; Center for Molecular Medicine and Genetics).

“The effects of selection pressures on the efficiency of placentation may stem from changes in nutritional demand, gestational length, number of embryos per pregnancy, uterine shape and maternal body constitution,” Dr. Wildman said.

“There may be more variability in placenta structure than any other mammalian organ,” said Dr. Goodman. A pioneer of molecular phylogenetics, Dr. Goodman rocked the scientific world in 1962 by suggesting that gorillas and chimpanzees are more closely related to humans than to other great apes and monkeys. Throughout the decades, he and his colleagues have built substantial evidence to show that the DNA of chimpanzees and humans makes them more than 99 percent identical.

With more than 1,000 students, the Wayne State University School of Medicine is the nation’s third largest medical school. Together with its clinical partners, the Wayne State University Physician Group, the Detroit Medical Center and other area health care providers, the school is a leader in medical research and patient care with emphases on cancer, women’s and children’s health, neurosciences, population studies and urban health.