Washington, March 18 : A new study has found that the surfacing of alternative nourishment resources, such as lactation, paved the way for mammals' loss of egg yolk nourishment.
The very first mammals on earth were reptile-like creatures that laid eggs.
The study has shown that the unique mammalian trait of nursing the young ones could have distanced our ancestors from egg-laying, as developing offspring were able to shift from a yolk to a milk diet.
"The reason we're known as mammals is because of our mammary glands," Live Science quoted researcher Henrik Kaessmann, an evolutionary biologist at the University of Lausanne in Switzerland, as saying.
"Nourishment with milk is a key feature of mammals. It's at the center of our story. And we wanted to know how that came about, how we came about," he added.
To have an in-depth understanding of how the distant ancestors of humanity and other mammals evolved from reptile-like creatures that laid eggs, Kaessmann and his colleagues looked at genes linked with eggs and milk.
There are three living types of mammals: placental mammals (humans, dogs, sheep, tigers, etc.), marsupial mammals (found in Australasia and South America, including kangaroos and possums), and monotremes (the duck-billed platypus and two species of Echidna).
The reproductive strategies of these three groups are very different. Placental mammals have long pregnancies and complicated placentas that provide nourishment to the embryo, followed by a relatively short period of lactation. Marsupials have a simpler form of placenta and much shorter pregnancies, followed by an extended period where the offspring is fed milk that changes in composition to meet the baby's altering nutritional needs. Monotremes-once a diverse group, but now restricted both in species number and distribution-have a much more reptilian beginning, as they lay eggs filled with yolk.
For the study, the scientists compared the genes of representatives from these different mammal lineages with those of chickens - an egg-laying, milkless control.
The authors found that there are similar genetic regions in all three mammalian lineages, suggesting that the genes for casein (a protein found in milk) arose in the mammalian common ancestor between 200 and 310 million years ago, prior to the evolution of the placenta.
Eggs contain a protein called vitellogenin as a major nutrient source. The authors looked for the genes associated with the production of vitellogenin, of which there are three in the chicken. They found that while monotremes still have one functional vitellogenin gene, in placental and marsupial mammals, all three have become pseudogenes (regions of the DNA that still closely resemble the functional gene, but which contain a few differences that have effectively turned the gene off).
The gene-to-pseudogene transitions happened sequentially for the three genes, with the last one losing functionality 30-70 million years ago.
Therefore, mammals already had milk before they stopped laying eggs. Lactation reduced dependency on the egg as a source of nutrition for developing offspring, and the egg was abandoned completely in the marsupial and placental mammals in favor of the placenta. This meant that the genes associated with egg production gradually mutated, becoming pseudogenes, without affecting the fitness of the mammalian lineages.
The evolution of milk reduced the need that mammal offspring had for the nutrients in the yolk and therefore eggs, the researchers suggest.
Eventually, marsupials and placentals abandoned egg-laying completely, leading genes linked with egg yolk to mutate and stop functioning over time, they added.
Indeed, the evolution of milk "seemed to have triggered the chain of events behind the complete loss of egg yolk genes," Kaessman explained.
"These findings shed light on the big question of when and how the transition from eggs happened in mammals," he added.
Kaessmann and his colleagues David Brawand and Walter Wahli detailed their findings online March 14 in the journal PLoS Biology.