Milk may be good for growing bones, but it also had a powerful effect on mammalian evolution. A new genetic analysis supports the idea that lactation arose before mammals gave up the egg-laying ways of their reptile ancestors. And milk may have helped drive the biological changes that made the switch to live birth possible.
Mammals first appeared some 200 million years ago. Over time, most of them replaced their shell-bound eggs to nurture the fetus in utero with a placenta. They also began rearing their young with milk. The living exceptions are monotremes, such as the platypus, which make milk but still lay eggs. Henrik Kaessmann wanted to nail down the genetic changes that made this transition possible and when they occurred.
Kaessmann, an evolutionary geneticist at the University of Lausanne (UNIL), Switzerland, and colleagues focused on three genes in chickens that code for proteins essential for egg-laying. They then searched for related sequences in the genomes of four mammals: humans, opossums, dogs, and platypuses. In humans, dogs, and opossums, all three genes were still present, but mutations had rendered them inoperative; the last one stopped working 30 million to 70 million years ago. One of the genes is still functional in platypuses, the team reports 18 March in PLoS Biology. That makes sense because monotreme eggs have less yolk than that typically found in bird and reptile eggs.
The researchers next looked at milk-producing genes. They found that platypuses share genes for milk production with other mammals. Because platypuses split from other mammals 180 million years ago, the find indicates that lactation arose in a common ancestor of both groups thought to have lived 200 million to 310 million years ago--well before the loss of egg-laying genes.
The findings confirm previous thinking that lactation was the driving force behind the downfall of the egg in mammals, as they no longer needed to rely on the egg as a source of nutrition for their young, says Jenny Graves, an evolutionary biologist at the Australian National University in Canberra. Peter Vogel, an evolutionary biologist at UNIL who was not involved in the study, says the transition makes sense because lactation confers numerous advantages. For example, the parents do not have to bring food items for the young back to a nest and can devote more time to nurturing fewer young, which allows for the development of larger brains in mammals.