For more than a century, biologists have wondered what the earliest animals were like when they first arose in the ancient oceans over half a billion years ago.
Searching among today's most primitive-looking animals for the earliest branch of the animal tree of life, 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 predators that oar their way through the world's oceans in search of food.
In a U.S. National Science Foundation-supported study published in the journal Nature, researchers used a novel approach based on chromosome structure to come up with a definitive answer: Comb jellies, or ctenophores (teen'-oh-fores), were the first lineage to branch off from the animal tree. Sponges were next, followed by the diversification of all other animals, including the lineage leading to humans.
Although the researchers determined that the ctenophore lineage branched off before sponges, both groups of animals have continued to evolve from their common ancestor. Nevertheless, evolutionary biologists believe that these groups still share characteristics with the earliest animals, and that studying these early branches of the animal tree of life can shed light on how animals arose and evolved to the diversity of species we see around us today.
"The most recent common ancestor of all animals probably lived 600 or 700 million years ago. It's hard to know what they were like because they were soft-bodied animals and didn't leave a direct fossil record," said Daniel Rokhsar, a University of California, Berkeley molecular and cellular biologist and co-corresponding author of the paper. "We're looking back deep in time where we have no hope of getting fossils, but by comparing genomes, we're learning things about these very early ancestors."
Understanding the relationships among animal lineages will help scientists understand how key features of animal biology, such as the nervous system, muscles and digestive tract, evolved over time, the researchers said.
