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Did you know that the creature which laid the basic body plan for most complex animals, including humans, emerged 700 million years ago? That animal had a front and a back, a top and a bottom. It was an inconspicuous animal, lived on the seafloor, and was the last common ancestor of bilaterians, a large group of animals with bilateral symmetry.
Invertebrates including molluscs, arthropods, worms, and insects, and vertebrates including pisces, amphibians, reptiles, birds, and mammals are bilaterians.
A study of 20 different bilaterian species including humans, mayflies, octopuses, sharks, and centipedes has found that over 7,000 groups of genes can be traced back to the last common ancestor of bilaterians. The study, led by researchers at the Centre for Genomic Regulation in Barcelona, was recently published in the journal Nature Ecology and Evolution.
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Ancestral gene specialisation in the last 700 million years
In the last 700 million years, animals have repurposed around half of the ancestral genes for use in specific parts of the body, especially the brain and reproductive tissues. Most ancient, conserved genes have fundamental, important jobs in different parts of the body, which is why the discovery that half of these were repurposed is surprising.
A series of serendipitous ‘copy paste’ errors during bilaterian evolution was responsible for the changes in half of the ancestral genes.
Early in the history of vertebrates, some tissue-specific genes emerged during two whole genome duplication events. One of the copies was used for fundamental functions, while the second copy was used as raw materials for evolutionary innovation. Several events of this kind occurred throughout the bilaterian evolutionary tree.
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How ancestral gene specialisation cooked up the animal kingdom
It is these ‘copy paste’ errors in genes that cooked up the animal kingdom.
In a statement released by the Centre for Genomic Regulation, Federica Mantica, an author on the paper, said genes are like a vast library of recipes that can be cooked up differently to create or change tissues and organs.
The two copies of genes, or alleles, for a particular trait can be compared to two copies of a recipe for paella. One can enjoy the original recipe, while evolution changes the extra copy of the recipe to make risotto instead. Both are European rice dishes.
Since all genes are copied twice, a large number of possibilities open up for evolution.
These events took place hundreds of millions of years ago, and their legacy lives on in most complex animals today, said Mantica.
Some interesting examples of new tissue-specific functions made possible by the specialisation of ancestral genes include: the TESMIN and tomb genes, which originated from the same ancestor; and FGF17, a gene playing a role in human cognition.
The TESMIN and tomb genes play an important role in spermatogenesis in both vertebrates and insects, and problems with these genes can affect sperm production and fertility in both mice (vertebrate) and fruit flies (insect).
Complex nervous systems were also formed due to the specialisation of ancestral genes. The scientists found certain genes in vertebrates which are critical for the formation of myelin sheaths around nerve cells which, in turn, are key to fast nerve signal transmission.
FGF17, another product of ancestral gene specialisation, is believed to play a crucial role in maintaining cognitive functions into old age.
Insect flight also arose as a result of ancestral gene specialisation. Certain genes in the muscles and epidermis of insects became specialised for cuticle formation, allowing them to fly.
Some genes in the skin of octopuses became specialised to perceive light stimuli, conferring the animals the ability to change colour and camouflage.
The study concluded that genes can lead to the development of new physical traits or abilities when they start acting in specific tissues, ultimately contributing to animal evolution.
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