When wondering about the origins of our brain, don't look to Homo sapiens, chimpanzees, fish or even worms. Many key components first appeared in single-celled organisms, long before animals, brains and even nerve cells existed. …
The finding is intriguing on its own, but much more significant when combined with a growing body of evidence that essential brain components evolved in choanoflagellates before multicellular life appeared.
In 2008, Xinjiang Cai of Duke University in Durham, North Carolina, discovered that M. brevicollis has the same calcium channels in its cells as those used by neurons. Then, in 2010, it emerged that M. brevicollis also has several proteins that neurons use to process signals from their neighbours.
And this year, Harold Zakon of the University of Texas at Austin and colleagues discovered that M. brevicollis has the same sodium channels that neurons use to pass electrical signals along their length.
Put together, these findings suggest that choanoflagellate cells have components for each of the three main functions of neurons: carrying electrical signals along their bodies, signalling to their neighbours with neurotransmitters, and receiving those signals. …
"The choanoflagellates have a lot of precursors for things we thought were only present in animals," says Fasshauer. Today, says Zakon, the nervous system seems "unbelievably complex", but evidence from these tiny organisms suggests it was built up from several simple systems, which evolved separately for different reasons. For instance, Fasshauer suspects M. brevicollis uses Munc18/syntaxin1 to secrete chemicals, much like neurons use it to release neurotransmitters.
So an ancient, primitive organism just happened to evolve spontaneously incredibly complex molecular machines that would be crucial components in the exquisite neuron designs in multicellular life. It would be like finding jet engine components in a wagon train. It seems that nothing in biology makes sense in the light of evolution.