In a link between the evolution lectures and our forthcoming animal physiology lectures there is news this week about the organ that may have been instrumental in leading to the success of the mammals.
'One organ, arising some 120 million years ago, stands above all others in the mammal's evolutionary ascent.'
Yes, it's the placenta. A paper published in Genome Research this week used timecourse microarray analysis over the lifetime of the placenta in mice to unravel the molecular and genomic changes that occur. At several intervals during mouse pregnancy, the researchers took genetic snapshots of the placenta that measured which genes were active. They then looked at the evolutionary history of the genes turned on by the growing placenta by comparing the DNA sequences of mouse genes to those found in other animals.
They report that two distinct evolutionary mechanisms were utilized during placental evolution in mice and human. When the organ starts growing, it switches on ancient genes shared by all animals many of them linked to cell growth and division. Midway through pregnancy, however, the placenta activates an entirely new set of genes, many found only in mice.
Ancient genes involved in growth and metabolism were co-opted for use during early embryogenesis, likely enabling the accelerated development of extraembryonic tissues. Recently duplicated genes are utilized at later stages of placentation to meet the metabolic needs of a diverse range of pregnancy physiologies. Together, these mechanisms served to develop the specialized placenta, a novel structure that led to expansion of the eutherian mammal, including humankind.