The first animal to appear on Earth has been identified by scientists. It was a strange jellyfish-like creature called a ctenophore - popular in modern aquariums.
All species evolved from the single common ancestor that lived up to 700 million years ago including sponges, insects, sharks - and us. Co-author Professor Daniel Rokhsar, of California University in Berkeley, said: "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. But we can use comparisons across living animals to learn about our common ancestors.
"It's exciting - 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."
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. The international team analysed strands of DNA called chromosomes to come up with a definitive answer.
They found ctenophores, pronounced 'teen'-a-fores', were the first lineage to branch off from the animal tree. They are commonly called comb jellies - and are found throughout the ocean today, from both poles to the equator and from the surface to the deep-sea.
Sponges were next followed by all other animals - including the lineage leading to humans. Simple single-celled life began about four billion years ago.
Lead author Dr Darrin Schultz, of Vienna University, said: "We developed a new way to take one of the deepest glimpses possible into the origins of animal life. This finding will lay the foundation for the scientific community to begin to develop a better understanding of how animals have evolved."
Most familiar animals, including worms, flies, molluscs, sea stars and vertebrates — and including humans — have a head with a centralised brain, a gut running from mouth to anus, muscles and other shared features. These had already evolved by the time of the 'Cambrian Explosion' around 500 million years ago when the major diversification of animals occurred.
Other bona fide animals, however, such as jellyfish, sea anemones, sponges and ctenophores, have simpler body plans. They lack a defined brain and may not even have a nervous system or muscles — but still share the hallmarks of animal life, notably the development of multicellular bodies from a fertilised egg.
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Prof Rokhsar said: "Traditionally, sponges have been widely considered to be the earliest surviving branch of the animal tree, because sponges don't have a nervous system, they don't have muscles, and they look a little bit like colonial versions of some unicellular protozoans. And so, it was a nice story: First came the unicellular protozoans, and then sponge-like multicellular consortia of such cells evolved and became the ancestor of all of today's animal diversity.
"In this scenario, the sponge lineage preserves many features of the animal ancestor on the branch leading to all other animals, including us. Specialisations evolved that led to neurons, nerves and muscles and guts and all those things that we know and love as the defining features of the rest of animal life. Sponges appear to be primitive, since they lack those features."
The other candidate for earliest animal lineage is the group of comb jellies, popular animals in many aquariums. While they look superficially like jellyfish — they often have a bell-like shape, although with two lobes, unlike jellyfish, and usually tentacles — they are only distantly related.
And while jellyfish squirt their way through the water, ctenophores propel themselves with eight rows of beating cilia arranged down their sides like combs. Along the California coast, a common ctenophore is the 1-inch-diameter sea gooseberry.
Each species has a characteristic chromosome number — humans have 23 pairs — and a characteristic distribution of genes along chromosomes. The researchers had previously shown that the chromosomes of sponges, jellyfish and many other invertebrates carry similar sets of genes, despite more than half a billion years of independent evolution.
This discovery suggested that chromosomes of many animals evolve slowly, and allowed the team to computationally reconstruct the chromosomes of the common ancestor of these diverse animals. Prof Rokhsar said: "At first, we couldn’t tell if ctenophore chromosomes were different from those of other animals simply because they’d just changed a lot over hundreds of millions of years.
“Alternatively, they could be different because they branched off first, before all other animal lineages appeared. We needed to figure it out."
The researchers joined forces to sequence the genomes of another comb jelly and sponge, as well as three single-celled creatures that are outside the animal lineage: a choanoflagellate, a filasterean amoeba and a fish parasite called an ichthyosporean.
Rough genome sequences of these non-animals already existed, but they did not contain the critical information needed for chromosome-scale gene linkage: where they sit on the chromosome. Remarkably, when the team compared the chromosomes of these diverse animals and non-animals, they found that ctenophores and non-animals shared particular gene-chromosome combinations, while the chromosomes of sponges and other animals were rearranged in a distinctly different manner.
Prof Rokhsar said: "That was the smoking gun — we found a handful of rearrangements shared by sponges and non-ctenophore animals. In contrast, ctenophores resembled the non-animals. The simplest explanation is that ctenophores branched off before the rearrangements occurred." The study is published in the journal Nature.
