For the first two billion or so years of life on Earth, the only organisms to exist were single-celled microbes. The evolution of biological complexity was so slow in those early days that scientists have dubbed one vast stretch of this period the ‘boring billion’. Evolution is not for the impatient.
What was the first animal on earth?
The first animals on earth – multi-cellular life-forms that breathe oxygen and consume other life-forms – have long been thought to have looked something like a modern-day sponge, a simple creature lacking organs, muscles or nervous system that is little more than a colony of cells arranged so as to be able to filter food from water.
Molecular clocks (which use the mutation rates of DNA to estimate the point in time that lineages shared a common ancestor) tell us that, whatever the first animal was, it was doing its thing about 800 million years ago. But fossil evidence is scarce around this time.
The fossil record doesn’t really get lively until about 580 million years ago when the enigmatic Ediacarans appear. These lobed, leafy, plant-like creatures seem to be mostly filter-feeders attached to the seabed, but some may have been free-living grazers. Chemical analyses show they contain compounds manufactured only by animals. But their relationships to modern animals – and, indeed, earlier ones – is a mystery. They might have been an evolutionary experiment that left no descendents.
If fossils cannot yet point us to the very first animals, genetics can get us tantalisingly close. By comparing the DNA sequences of living animals, geneticists can reconstruct evolutionary relationships right back to the point at which the first animal lineage split into two.
Several attempts have been made, with some studies suggesting that one of the two lineages gave rise to sponges and the other to all other animals. This would support the idea that sponges appeared before other animals. Other studies, though, point to the comb jellies (iridescent, jellyfish-like creatures that are not jellyfish, but belong to a separate group) as being the ones that first went off on their own, which would mean that the first comb jelly came into existence before the first sponge.
Equipped with muscles and nervous systems, modern comb jellies are far more complicated than sponges. But it doesn’t follow that the first ones would have been. After all, they have undergone hundreds of millions of years of independent evolution since then. Might it have looked something like a sponge?
There’s also the question of the identity of the simple animal life-form that existed before that first lineage split into two. Whatever it was, it left quite a legacy. Everything that creeps, crawls, slithers, scuttles, gallops, hops and flies owes its existence to this mystery ancestor.