We take for granted today that the standard pregnancy takes place entirely within the womb and typically lasts about 9 months. This makes evolutionary sense; we can carry our vulnerable young in the safety of the womb rather than having to leave it exposed in a nest or a pouch. But this is actually a phenomenon unique to mammals, and is the result, not of small genetic mutations that impacted mammals over time, but of a rapid genetic rewiring that took place over 100 million years ago. A study conducted here at Yale that was published on September 25th, 2011, shows that the evolution of pregnancy in mammals owes much to transposons, otherwise known as “Junk DNA.”
The Yale professors examined uterine cells that are usually associated with placental development in opossums, armadillos and humans. Opossums are marsupials that give birth a mere two weeks after conception, while armadillos and humans, both mammals, have 9-month pregnancies and highly evolved placentas. The results of these comparisons showed that there were 1500 uterine genes that were unique to the placental mammals. This proves that these differences in uterine structure and pregnancy terms have their root in genetics.
About 13 % of the genes that were found in the placental mammals were very near on the genome to a particular kind of transposon. Transposons are pieces of genetic material that replicate within the host genome and used to be called Junk DNA because no one really understood their purpose. In a sense, transposons are parasitical because they multiply in the genome and affect it in a way that makes it carry out entirely new functions. These transposons in particular activated genes related to pregnancy and thus altered maternal-fetal communication. The transposons made the uterine cells more sensitive to progesterone, a birth-related hormone, and encouraged the cells’ development into the placenta. This resulted in a longer pregnancy term for mammals.
Ultimately, the take-away from this study is the fact that this was not a long-term process, it was a large-scale morphological change done relatively swiftly through a cut-and-paste operation that affected massive areas of the genome.
"Invasion of Genomic Parasites Triggered Modern Mammalian Pregnancy, Study Finds." Science Daily. 25 Sept. 2011. Web. 27 Sept. 2011.
Lynch, Vincent J. "Transposon-mediated Rewiring of Gene Regulatory Networks Contributed to the Evolution of Pregnancy in Mammals." Nature. 25 Sept. 2011. Web. 27 Sept. 2011.
"Junk DNA Gave Us the Modern Uterus, in a Giant Genetic Cut-and-Paste Operation." Discover Magazine. 27 Sept. 2011. Web. 27 Sept. 2011.