By analyzing chromosomes a team of researchers answers the question of what the first animals looked like, from which the diversity of animals in the world today was created.

These first animals were marsupials and not sponges, they say.

For more than a century biologists have wondered what the first animals were like when they first appeared in the ancient oceans half a billion years ago. Searching among today’s primitive-like animals, scientists gradually narrowed the possibilities down to two groups: sponges, which spend their entire adult lives in one spot filtering food from seawater, and comb jellies, voracious eaters. predators that move through the world’s oceans in search of food.

In a new study, published in the journal Nature, researchers use a new approach based on the organization of genes on chromosomes to provide the answer: marsupials were the first lineage from which the branches in the animal tree arose. Sponges followed, and then the diversification of all other animals, including the lineage leading to humans.

“The most recent common ancestor of all animals probably lived 600 or 700 million years ago. It’s hard to know what they looked like because they were soft-bodied animals and left no direct fossil record. But we can use comparisons between animals that exist today to learn about our common ancestors,” says Daniel Roxar, professor of molecular and cellular biology at the University of California, Berkeley and one of the authors of the paper.

“It is exciting. We look back deep in time, where we have no hope of finding fossils, but by comparing genomes, we learn things about these very early ancestors“, he adds.

Most known animals called bilaterians, including worms, flies, molluscs, vertebrates, and even man, have a head with a central brain, a gut that extends from the mouth to the anus, muscles, and other common features that they already had evolved since the time of the famous “Cambrian explosion”, when the animal kingdom was created.

Other animals, however, such as jellyfish, sea anemones, sponges and cetaceans, have simpler body designs. They lack a defined brain and may not even have a nervous system or muscles, but they still share the characteristic features of animals, most notably the development of multicellular bodies from a fertilized egg.

The evolutionary relationship between these different creatures has been controversial.

With the development of DNA sequencing biologists were able to compare the gene sequences shared by animals to construct a family tree showing how animals and their genes evolved over time.

However, the question remained as to whether sponges or cetaceans were the first branch of animals.

Understanding the relationships between animal lineages will also help scientists study how key features of animal biology have evolved over time, such as the nervous system, muscles and digestive system, the researchers report.