Genome reconstruction of first mammal suggests it had 38 chromosomes: ScienceAlert

Scientists don’t know much about what the first mammal looked like, but they do know that it lived about 180-250 million years ago, and that every mammal on Earth – from blue whales to platypuses – is descended from it.

But thanks to new research, we now know what his genome looked like.

An international team of scientists has computationally pieced together a possible genome for the common ancestor of mammals, working backwards from the genomes of 32 living species.

The analysis included a wide range of species from all three mammal types, including narwhals, pangolins, bats and humans for the placental mammals, Tasmanian devils and wombats for the marsupials, and the egg-laying platypus.

Chickens and Chinese alligators were used as a non-mammalian comparison group.

The researchers reconstructed the complete set of chromosomes into 16 nodes that stretch back to the common ancestor of all mammals. (A node represents the last common ancestor between two distinct genetic lines; it is the point where the phylogenetic tree splits into multiple branches.)

The researchers concluded that the species at the beginning of the mammalian phylogenetic tree probably had 38 chromosomes.

It shared nine of the smallest chromosomes with the common ancestor of mammals, birds and reptiles, which is a step even further back in the tree.

“This remarkable finding demonstrates the evolutionary stability of the order and orientation of genes on chromosomes over an extended evolutionary time frame of more than 320 million years,” says senior author and evolutionary biologist Harris Lewin.

Many of these highly conserved regions contain genes involved in developmental functions.

The researchers looked at how chromosomes broke, combined, deleted, duplicated or shifted over time.

Chromosome segments strongly affected by rearrangements are called “breakpoints,” a rich source of genetic variation that plays a role in separating species through evolution.

The highest breakpoint rate was observed when carnivores – marsupials and placental mammals that give birth to live young – separated from egg-laying monotremes.

“Our results have important implications for understanding mammalian evolution and for conservation efforts,” says Lewin.

It is possible that the earliest mammal was somewhat similar to the tiny rat-like mammal called Morganucodon, which lived about 200 million years ago and laid eggs. Its fossil was discovered in a limestone crevice in 1949 in Wales in the United Kingdom.

This genus is related to living mammals, but is not considered a common ancestor, making it a sister group to the order of mammals.

Another mammal sibling is the rodent A rattlesnake genus. Fossils found in Africa and North America are too specialized to be a common ancestor of all mammals, but they would have lived around the same time as the first mammal species.

This work was published in Proceedings of the National Academy of Sciences.

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