by Matthew Cobb
Aristotle thought they were plants; for centuries, we have used them to scrub ourselves clean, while millions of kids round the planet think they wear square pants and play with stupid starfish. Nevertheless, sponges are in fact the oldest living animals, one of the most amazing success stories on the planet.
Argument rages over exactly how old the earliest sponge is. Some fossils that are claimed to be sponges have been dated back to around 710 MY ago, but not everyone accepts these. Everyone agrees, however, that they were around shortly before the Cambrian Explosion, around 560MY.
They now cover every part of the ocean, and can account for up to 75% of the biomass on the floor of the Antarctic ocean. 15,000 species have been identified, most of them marine, but there are around 150 freshwater species. There are three main extant groups, and – confusingly – three main body forms, which don’t overlap (i.e. the body forms don’t form distinct evolutionary lineages).
One group of sponges – the Archaeocyaths – flowered briefly (metaphorically speaking) during the Cambrian, but disappeared after around 15 MY. Most sponges are filter feeders, but a few deep sea sponges are carnivorous, spiking tiny crustaceans on their spicules and then absorbing their body contents.
Sponges have no neurons, no muscles, no cell-cell junctions, no gut, no front/back, no reproductive organs.
So what makes them animals?
Studies of DNA show that all the animals – “metazoa” – form what is called a monophyletic group. In other words, all us animals share a common ancestor which is not shared by any other life form. Animals evolved only once, and the earliest branch is the sponges, says the data.
But strictly speaking, that doesn’t make them an animal. They could be some multicellular non-animal, which branched off before the rest of us started on our animally way.
However, recent studies have revealed some amazing things about sponges that clearly place them with the animals – they share fundamental characters with us that no other life-form shares. We can see in the sponge the beginnings of our own way of organizing our bodies and behaving.
They have a simple immune system – a sponge will accept a graft of its own flesh, but will reject that of another sponge. This rejection can be stopped by using an immunosuppressant widely used in medicine. In other words, we share a biochemical pathway that the body uses to distinguish self and non-self.
They show coordinated movement – Ephydatia muelleri, a small sponge, “sneezes” (rather slowly!) when it is agitated or has inedible ink poured over it. This movement can be induced by pouring known animal neurotransmitters over the sponge, showing the sponge cells are sensitive to stuff that we use to activate our neurons.
They may not have neurons, but they have the tiny ion channels that are required to make a neuron work. Express these genes in another organism, and they work.
They have a simple form of the genes required to make neurons. Express these genes in a toad or fly and they start to make neurons there.
This is exactly what we would expect would have happened at the earliest stages of animal multicellularity. Animals didn’t suddenly appear as running, flying, swimming things. Our earliest, multicellular ancestors would have been perfectly adapted to their environment, but would also have used simple forms of cell-cell communication to build their bodies, and to respond to that environment.
Over the vast depths of evolutionary time, those methods were honed and shaped by natural selection, eventually leading to the stupendous array of animal life that the planet has carried, including me, typing these words to you.
But that doesn’t make sponges “primitive”, or in any way lesser beings. They have been on our planet for longer than any other animal – around 10,000 times longer than our species. They have survived repeated mass extinction events, and that suggests they will probably be around for at least another 600 MY. Not bad for an animal that Aristotle took for a plant.