How mammals started nursing and stopped laying eggs

This video is called Critical Analysis of Evolution vs. Creation – Part 2.

From LiveScience:

Mom’s Milk Helped Mammals Drop Egg-Laying

By Charles Q. Choi, Special to LiveScience

posted: 17 March 2008 ET

The very first mammals were reptile-like creatures that laid eggs.

It turns out the ability to nurse their young — a trait unique to mammals — could have led our distant ancestors away from egg-laying, as developing offspring were able to shift from a yolk to a milk diet.

All mammals have at least four physical traits in common. We all possess hair at some point — even whales and naked mole rats [see also here]. We all have three bones in our middle ear that help amplify sound. We all possess a neocortex in our brains, a structure responsible for higher brain functions. And all mammal species can produce milk.

“The reason we’re known as mammals is because of our mammary glands,” explained researcher Henrik Kaessmann, an evolutionary biologist at the University of Lausanne in Switzerland. “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.”

To better understand how the distant ancestors of humanity and other mammals evolved from reptile-like creatures that laid eggs, Kaessmann and his colleagues investigated genes linked with eggs and milk.

There are three major types of mammals alive today. These include humans and other placentals, kangaroos and other marsupials, and duck-billed platypuses and the scant few remaining egg-laying monotremes. The scientists compared the genes of representatives from these different mammal lineages with those of chickens (which are egg-laying and milkless, naturally).

DNA accumulates mutations over time, serving like a clock. The new findings suggest the genes for “casein” proteins found in milk arose in the common ancestor of all mammals between 200 million and 310 million years ago.

In contrast, genes for proteins called vitellogenins that provide the nutrients found in egg yolk were gradually lost in all mammals, except the monotremes, just 30 million to 70 million years ago. (Since monotremes still lay eggs, they naturally kept some yolk proteins.) The three genes for vitellogenins found in the chicken all became mutated, useless “pseudogenes” in placentals and marsupials, and just one functional vitellogenin gene is seen in monotremes.

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.

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 said.

Kaessmann and his colleagues David Brawand and Walter Wahli detailed their findings online March 17 in the journal PLoS Biology.

See also here. And here.

Platypus genome and evolution: here.

The therapsid Gorgonops: here.

The new species, named Tiarajudens eccentricus, was part of a group of ancient animals called therapsids, many of which are relatives of today’s mammals: here. And here.

Naked mole rats are becoming more popular in research laboratories, where the seemingly invulnerable rodents have surprised scientists with their ability to live up to 30 years and their potential to offer insights into human health. They’re being used to study everything from aging to cancer to strokes: here.

3 thoughts on “How mammals started nursing and stopped laying eggs

  1. Public release date: 7-May-2008

    Contact: Louisa Wright
    European Molecular Biology Laboratory
    Platypus genome sequence published
    Platypus genetic blueprint reveals the early history of mammals

    UK-based researchers at the Medical Research Council Functional Genomics Unit in Oxford and the European Molecular Biology Laboratory’s European Bioinformatics Institute in Cambridge have revealed the genetic makeup of the one of the world’s strangest mammals. They have analysed the DNA blueprint of the platypus, one of only a few surviving monotremes which, of all mammals, are the most distantly-related to humans. The platypus, a female nicknamed Glennie, was sequenced by scientists at the Genome Sequencing Centre of Washington University School of Medicine, USA as part of an international research collaboration including scientists from the UK and Australia. The analysis is published in the 8 May issue of Nature.

    The platypus is thought to have diverged from a common ancestor shared with humans approximately 170 million years ago. The species has many features that are unique to mammals; for example it has fur and rears its young on milk. However, it also shows reptile-like characteristics; the females lay eggs and the males produce venom. Some features, such as a specialised system in the platypus bill that uses electricity to detect food under water (electro-reception), are unique to monotremes. The researchers found that these diverse characteristics are mirrored by a patchwork of genes resembling those from reptiles, birds and other mammals. Lead researcher Chris Ponting from the MRC Functional Genomics Unit at the University of Oxford said “The platypus genome is extremely important because it is the missing link in our understanding of how we and other mammals first evolved. This is our ticket back in time to when all mammals laid eggs while suckling their young on milk. It also provides an essential background to future advances in understanding mammalian biology and evolution.”

    The researchers searched the genome for DNA sequences that are unique to the monotremes, as well as those known to be involved in venom production, electro-reception and milk production in other species. They discovered that platypus venom is a cocktail of proteins that originally had very different functions. Amazingly, the same proteins are found in reptile venom even though platypus and snake venom evolved independently. They also found that the platypus has many more sex chromosomes – the organised structures into which DNA is packed that determine sex – than do humans. The platypus has ten sex chromosomes, compared with our two. Furthermore, the gene sequences responsible for determining sex are more similar to those in birds than in mammals. Ewan Birney, who led the genome analysis performed at the European Bioinformatics Institute, commented “The platypus looks like such a strange blend of mammalian, bird-like and reptilian features and now we know that the genome is an equally bizarre mix of all of these. It’s much more of a mélange than anyone expected.”


  2. Pingback: Mammal-like reptile’s brain, new research | Dear Kitty. Some blog

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