Sponges don't ponder about the meaning or origin of life. But in some ways they are better at the whole life thing than we are. They have lived for millions more years, surviving on the sea floor by taking in nutrients through their porous bodies.
To our eyes, they look almost laughably simple. They have no brain, and indeed no nerve cells. But they get along just fine without either.
Sponges' brainlessness might even be a positive thing, something that evolution has favoured. Some scientists now believe that they once had a brain, or at least something much like it, but then got rid of it. And they are not the only ones. To us the brain seems like a necessity, but it may be that some animals actually do better without them.
A brain is what you get when many nerve cells, known as neurons, cluster together into one big lump. Many organisms do not have true brains, but rather a "nerve net" of neurons scattered through their bodies. However, sponges do not even have that.
Complex brains were in place as early as 520 million years ago
The origin of our brain starts almost four billion years ago, when life first sprang into being. Our earliest ancestors were single-celled organisms, and it would be another few billion years later before more complex organisms appeared. It's not clear whether they had any nerve cells.
The oldest known fossil with a complex brain is about 520 million years old. This was a time when life became much more abundant and diverse, often referred to as the Cambrian explosion.
Discovered in China, the animal looked like a woodlouse with claws. It seems to have had an elaborate brain-like structure consisting of a fore-, mid- and hind-brain, all of which had specialised neural circuits.
This suggests that complex brains were in place as early as 520 million years ago. But they may not have stayed.
In their ancient evolutionary past, sea sponges did have neurons, according to Frank Hirth of Kings College London in the UK. He says the sponges have experienced "evolved loss" of these structures, an argument he laid out in a paper in the journal Brain, Behaviour and Evolution in 2010.
There is plenty of precedent for this. Many species have lost seemingly vital organs. For instance, crustaceans living in dark caves are losing their eyes.
The key piece of evidence that sponges have lost their brains comes from phylogenetics: the attempt to figure out how all the different animal groups are related to each other. Researchers have drawn up a "tree of life", just like a family tree, showing the relationships.
Sponges were long thought to be the sister group to all other living animals, having branched off early on. This would imply that, of all the living animals, sponges are the most similar to the ancestral animals.
The strange thing is that comb jellies have an intricate nervous system
This was thrown into disarray by research published in the journal Nature in 2008.
Researchers analysed snippets of genes from many organisms, including a second group of marine animals called comb jellies or sea gooseberries. These have now taken the sponges' place as the sister group to all other animals, and our best representation of the ancestral animals.
The strange thing is that comb jellies have an intricate nervous system. This means that their ancestors, which must also have been the ancestor of sponges, probably did too. If that's true, somewhere along the way sponges lost their nerves.
There is some genetic evidence to support that. Sponges have many of the genes needed to build a nervous system, says Joseph Ryan of the University of Florida in St. Augustine. But they do not do so.
Sponges are clearly masters at what they do
Getting rid of your brain sounds like a bad idea. So why would sponges ditch theirs?
First of all, the brain eats up an enormous amount of energy. In humans, up to 20% of our energy is spent feeding our brain.
Meanwhile sponges are clearly masters at what they do: filtering water and picking out only the useful, nutritious particles. Adding a nervous system might not help with that.
"If you are sitting on the sea bed and just filtering food that comes along, you don't need a brain," says Hirth. "It would be a waste of energy and you wouldn't be able to maintain this energy demand."
"For a long time we thought that sponges are primitively simple, that they never had a nervous system at all," says Ryan. "It may take a while to see that [idea] shifting."
Sea squirts simplify their brains during their lifetimes
Sponges may not be the only creatures that have lost, or at least simplified, their nervous systems. Some parasites, such as fluke worms that have only very basic neural cells, also seem to have lost complexity compared to close relatives, says Hirth. "One would assume that their parasitic lifestyle does not require a complex brain."
Another group called the placozoa, simple animals that are close relatives of sponges, have also lost their nervous systems according to Ryan and Hirth.
Meanwhile, sea squirts simplify their brains during their lifetimes. The larvae have well-developed brains, but once they settle on the sea ground and metamorphose into adults, these structures are reduced.
Still, not everyone believes that these animals have lost their neurons and brains.
Neuroscientist Leonid Moroz, who is also at the University of Florida in St. Augustine, believes that sponges never had neurons to start with.
There are no fossils to indicate they ever had neurons
They simply do not need any, he says, and nor did their ancestors. "We have 500 million years of the same ecology, the same filtering behaviour, with limited types of movement."
Neither the sponges nor the placozoa have any genes that Moroz would categorise as neuronal. And there are no fossils to indicate they ever had neurons, he points out.
The question then becomes how the comb jelly could have evolved such an intricate nervous system when their ancestors, and the ancestors of sponges, did not have one.
The answer, Moroz believes, is that the brain evolved more than once.
When the comb jelly genome was fully sequenced in 2013, researchers found that genetically they are unique. Moroz calls them "aliens". "They have a completely different molecular make-up from any other animal on our planet," he says.
Yet somehow, they had also created a nervous system. "Nature shows us that there is more than one way to make neurons," says Moroz. "We can design neurons using completely different principles. Nature is much more innovative than we think."
There is precedent for organs evolving more than once. Some organs, such as eyes, are known to have evolved many times over in different species. For instance, the eyes of octopuses are quite different from ours. This shows, says Moroz, that it's clearly possible to make a complex structure more than once.
This argument came to a head at a meeting at the Royal Society in London, UK in March 2015. Moroz and Hirth presented their differing viewpoints, each backed up by published research. Each remains adamant that they are correct.
Right now we don't know either way, says Angelika Stollewerk of Queen Mary University of London in the UK. With the evidence we have, either story is possible: maybe the nervous system has evolved twice, or maybe it was reduced in sponges.
It won't be easy to settle this disagreement. Quite possibly it will take high-quality fossils of early sponges and comb jellies to settle whether or not their ancestors had brains.
Either way, the tale of the sponges' brains is a reminder that one of the standard myths about evolution is wrong.
Many of us have the idea that evolution takes simple organisms and makes them more complex. It does sometimes do that, but it can also do the exact opposite and simplify things – and sometimes it keeps animals virtually unchanged for millions of years.
Sponges are a case in point. They have survived, without a thought or even a brain to think with, for hundreds of millions of years. They have never needed to get any more complicated, and intelligence wouldn't have helped them.
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