Somewhere between single-celled organisms and human beings, brains evolved. Just why and how is still shrouded in mystery.
The human brain is said to be among the most complex organizations of matter in the known universe. This complexity didn’t just spring from nothing. Like Legos, the molecular parts of what were eventually assembled into brains were mostly there, with a genetic reach that extends back in time to species that existed when the tree of life was just a wee sapling. In some cases, those Legos bore only faint resemblance to their modern forms - while others were so fundamental that their modern counterparts seem very familiar.
The most frustrating thing about telling the origin story of the brain is that fossils don’t possess enough detail in their structure to allow direct observation of molecules like proteins and neurotransmitters. The best we can do is to examine the descendants of creatures that first swam the earth to see whether we can discern a dim echo of our modern brain components.
One way scientists do this is through a genetic analysis of these descendants. By comparing the human genome with other species, biologists can detect patterns in the genes - allowing the identification of similar genes to those we possess, and allowing an estimation of how much a gene may have changed over millions, and even billions of years. It’s an estimate limited by the fact that the comparison is made against species that are themselves ancestral – it's a bit like comparing your nose to that of your distant cousin to learn what your great grandmother's was like.
A molecular approach to the brain’s origin is also limited by the fact that a protein might not have done exactly the same thing in ancient creatures that it does in a modern human brain. Examples of this are the ion channels, which early on may have been more important for maintaining osmotic balance than in generating electrical signals. But the Lego bricks were there in bacteria, billions of years ago, and later in paramecia, which developed the ability to generate and discharge electrical gradients.
Sponges (and choanoflagellates) also possess some of the same genes that are found in neurons – including neurotransmitter receptors and the protein scaffolds that give structure and function to the dendrites of neurons. And the cells of sponges can coordinate, through generating action potentials and by releasing chemicals that are like neurotransmitters. These are great building blocks, but it's still short of a brain.
Comb jellies evolved at about the same time as sponges, and possessed one of the first true neural networks capable of coordinated action, with another important twist – the orientation of that action along a motion axis. Is this a “brain”? That’s arguable – it would never compose a sonnet, but it would certainly be capable of many adaptive functions (note however that some important bits related to development are also missing, suggesting the possibility of a separate lineage). These developments foreshadowed the idea of cephalization, or the evolution of heads in which to keep the enlarging clusters of neurons that would become a brain. An example of this is the flatworm, which not only had a head-like region, but also eye spots and bilaterally symmetrical nerve cords that were a departure from the radially symmetric nerve nets. Fish extended cephalization in several ways, including Haikouichthys, which developed more complex brain structures, including what appear to be advanced senses and a prototypical vertebral column. Placoderms, a type of armored fish, are thought to have possessed the first mylenated axons, so important in speeding signals around our modern noggins.
The adaptations of modern brains (many of which I have skipped over) did not arise as a neat package - ion channels, synaptic proteins, the ability to generate active electrical signals, and the development of an enclosed nervous system arose at different times in life’s history. As suggested in the findings related to comb jellies, it’s probably true that some variants of the molecules that serve the essential functions of the brain evolved and died out, perhaps many times – we’re talking billions of years, after all. But enough of these features were adaptive that new species would carry them forward as part of our brain's molecular legacy. It's a legacy that we're rewriting every day as we learn more about it.
The brain is one solution to the challenges of living on planet earth. It’s a fantastic and highly adaptable solution that we see extensions of even in our most distant animal cousins. And we are fortunate that this solution is one that allows us the privilege of comprehending with wonder the colony of 86 billion cells that gradually arose from an ancient ocean and became conscious of itself.
[Note: Part II of this post takes a look at some misconceptions of evolution, including the risky notion of the human brain as a "capstone" of evolution]
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