The most complex animal on earth

Radiolab, my favourite popular science podcast, recently published an episode called “cellmates”, where they explain how life on earth went from being populated only by simple microscopic organisms to being filled with the complex, multicellular beings that surround us today. It’s quite a remarkable story, and I really recommend listening to the story, but here’s a short recap: for billions of years, life on earth was limited to simple, microscopic organisms. These organisms never developed more complexity, because they didn’t have any resources that would allow them to become bigger. Then, by a miraculous coincidence, two different organisms fused together into one and became a new organism. One of these were mitochondria, a tiny organism with the ability to use oxygen to generate electricity. This electricity provided energy so that the fused organism could build more complex structures, and all of the sudden life developed from simple microscopic beings into all the plants, animals, fungi etc that we have around us today.

This got me thinking, is there a limit to how complex an organism can be, what is the most complex organism we have on earth? Something really big maybe, like the blue whale? Or maybe humans? Nope, the answer is quite surprising. the most complex organism is a water flea, no bigger than 3mm long.

Meet daphnia pulex

Daphnia pulex
“Daphnia pulex”, watercolor on paper, 2016.

The water flea daphnia pulex has 31.000 genes, 25% more than us humans. It is the most common type of water flea (although they are actually crustaceans) and can be found in fresh water, often in lakes or ponds. It is the first crustacean to have it’s genome sequenced. More than a third of the daphnia pulex’s genes are completely unique and has never been seen in any other living organism. So why do they need so many genes? Well, the water flea has a pretty amazing ability to adapt to different situations and environments. It can transform it’s body in response to stress and develop different protective features like helmets, spines and even neck-teeth (!). Because of it’s adaptability to different environments, daphnia pulex is very useful for studying the effects of environmental changes and pollution.


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