Which animals came first? For more than a century, most evidence suggested that sponges, immobile filter-feeders that lack muscles, neurons and other specialized tissues, were the first animal lineages to emerge. Then, in 2008, a genomic study pointed to a head-scratching rival1: dazzling, translucent predators called comb jellies, or ctenophores, with nerves, muscles and other sophisticated features.
Ancient sea jelly makes tree of life wobble
That single study ignited a debate that has raged for nearly 20 years, sparking fierce arguments about how complexity evolved in animals. But after dozens of studies — some of which analysed and reanalysed the same data and reached different conclusions — the debate has become entrenched, some researchers say.
“Where it might have been healthy for people to engage with curiosity and an interest in finding the truth together, it became a battle,” says Nicole King, an evolutionary biologist at the University of California, Berkeley, who co-authored a paper last November that landed cautiously on ‘team sponge’2.
She has since asked to retract the paper because of flaws identified after its publication, and is reconsidering whether she wants to be part of the debate in the future. Scientists, including King, argue that a different approach is needed: one in which researchers from both sides work together to answer the question.
Fresh ideas — and attitudes — would catalyse progress, they say. “We must think out of the box,” says Leonid Moroz, a neuroscientist at the University of Florida in Gainesville, whose work has supported comb jellies as the lineage at the root of the animal family tree3.
The first animals emerge
Around 600 million to 800 million years ago, radically different organisms emerged. Instead of consisting of lone cells, like all previous life did, these creatures were formed of multiple, interacting cells. Multicellularity was so successful that it sparked an explosion in innovative body forms and new ways to sense and respond to environments.
In an evolutionary blink of an eye — perhaps within tens of millions of years — five major groups of animals appeared. As well as the ancestors of modern-day sponges and comb jellies, there were placozoa (now represented by blob-like marine invertebrates); cnidarians (modern members of which include jellyfish and sea anemones); and bilaterians that show mirror-image body symmetry in early development that would give rise to invertebrates, including starfish, snails and spiders, and vertebrates, including humans (see ‘Tree of life — now with two options’).
Fossil evidence of the earliest animals is sparse and hard to decipher — a porous cavity here or a branching tube there. Identifying the first animal lineage, along with knowledge of its modern-day descendants, is another way to gain insight into these early creatures. “Knowing this will tell us something, not everything, about what those first animals might have looked like,” says Max Telford, an evolutionary biologist at University College London. Evolutionary biologists sometimes call this first animal the ‘sister’ to other animal groups, because it shares a common parent with all of them.
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