2023-05-17 17:10:41
During the vast majority of its history, life on Earth was represented only by microorganisms. Bacteria and unicellular microbes that, for more than 3,000 million years, dominated a planet that was certainly boring, devoid of plants and animals of any kind. It is such a huge space of time that it is difficult to assimilate.
However, around 700 million years ago something began to change, and various groups of these microorganisms took their own path up the tree of life, independently evolving into more complex ‘something else’: the first animals. multicellular, from which later arose the immense variety of life forms that we know today.
For more than a century, biologists have wondered what those first animals were like. What they looked like and how they ‘worked’ when they arose in the planet’s first oceans. Without the possibility of obtaining fossils that allow them to be studied directly, scientists have been tracing the oldest-looking animals of today until they narrowed the possibilities to just two groups: sponges, which spend their entire adult lives in the same place, filtering food from the water sea; and comb jellyfish, voracious predators that paddle across the world’s oceans in search of food.
Now, in a new study just published’Nature‘, a team of researchers from the University of California at Berkeley and Santa Cruz, the University of Vienna, and the Monterey Bay Aquarium Research Institute (MBARI) have finally come up with a definitive answer: comb jellies. , or ctenophores, were the first to branch off the animal tree. Sponges were next, followed by the diversification of every other animal we know of, including the lineage that led to humans.
However, although the researchers determined that the ctenophores were the first, both groups (ctenophores and sponges) continued to evolve from their common ancestor. Biologists believe that these groups still share characteristics with their very distant ancestors today, and that studying those early branches of animal life may shed light on how they went from there to the immense diversity of species we see all around us today.
“The most recent common ancestor of all animals -explains Daniel Rokhsar, co-author of the study- probably lived 600 or 700 million years ago. It is difficult to know what shape they were because they were soft-bodied animals and did not leave a direct fossil record. But we can use comparisons between living animals to learn about our common ancestors. It’s exciting: we’re looking back in time, to a place where we have no hope of getting fossils, but by comparing genomes, we’re learning things about these very early ancestors.”
Understanding the relationships between animal lineages will help scientists understand how key features of animal biology, such as the nervous system, muscles, and digestive tract, evolved over time.
“We developed a new way to take one of the deepest possible looks into the origins of animal life,” says Darrin Schultz, lead author of the study. This finding will lay the groundwork for the scientific community to begin to develop a better understanding of how animals have evolved.”
What is an animal?
Most familiar animals, from worms, flies, mollusks, starfish, and vertebrates, including humans, have a head with a centralized brain, a gut that runs from the mouth to the anus, muscles, and other features. shared structures that had already evolved at the time of the famous “Cambrian Explosion” some 500 million years ago. Together, all these animals are called ‘bilaterals’.
However, other animals, such as jellyfish, sea anemones, sponges, and ctenophores, have simpler body designs. These creatures, in fact, lack many of the bilateral characteristics. For example, they don’t have a well-defined brain and don’t even have a nervous system or muscles, but they still share some important characteristics of animal life, notably the development of multicellular bodies from a fertilized egg.
The evolutionary relationships among these various creatures—specifically, the order in which each of the lineages branched off the main trunk of the animal tree of life—has been controversial, but with the rise of DNA sequencing, biologists were able to finally compare the sequences of genes shared by all to build a family tree that illustrates how both they and their genetic makeup evolved over time since the emergence of the first animals in the Precambrian Period.
A gene map
In their study, in fact, the researchers made a ‘map’ of those genes that are always found together on a single chromosome and in all animals, from humans and mice to crabs and corals. And in this way they managed to show that the ctenophores are the ‘sister group’ of all other animals.
The team describes the event that happened 700 million years ago as a genetic fork. A solitary single-celled organism, they explain, the ancestor of all animals, was traveling that road with its two descendants and came across a branch. The first of his ‘children’, destined to become the comb-jellyfish we know today, took a path. As it evolved, the genes on its chromosomes stayed in the same order, without changing much.
The other ‘son’, who would first become a sponge and then all the animals we know today, took the other path. Many of the genes on his chromosomes rearranged and fused together. And because these rearrangements are irreversible and are passed down from generation to generation, they can be detected even today. By tracking them, in effect, the team found clear evidence that comb jellies, and not sponges, were the ‘sister group’ to all other animals.
“The fingerprints of this ancient evolutionary event – explains Schultz – are still present in the genomes of animals hundreds of millions of years later. This research helps strengthen the foundation of our understanding of the genetics of animal life. It gives us context to understand what makes animals animals. This work will help us understand the basic functions we all share, such as how they perceive their environment, how they eat and how they move.”
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