How Did Brains Evolve?

 

Humans have asked where we come from for thousands of years, across all cultures. But only recently have we started to address the mystery of the evolution of the human brain — the organ that’s the source of those existential questions, not to mention our evolutionary success itself.

Our brains are each made of billions of cells, called neurons, that link together to form living circuit boards that control everything from our thoughts to our behaviors to the rhythm of our breathing. We aren’t the only creatures with brains, but our brains are unique in terms of their size relative to our bodies, and in terms of their complexity. Just how did incremental changes to ancient animal brains, over millions of years, eventually result in this most sophisticated of living, computing machines?

Unfortunately, the soft consistency of brain tissue has robbed us of our ability to directly reconstruct the origins of this defining human feature. Bones, buried in the right geological layer, can turn into fossils that last eons. But brains disintegrate quickly, leaving almost no trace behind.

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While fossils can’t show us exactly what ancient brains looked like, fossilized skulls can hint at what was required to protect and house ancient brains. Similarly, though the common ancestors of most species are extinct, the many surviving species that populate the planet, known as extant species, are useful for inferring the incremental changes that advanced the brain forward. For example, amongst extant vertebrates (animals possessing spines and spinal cords), it’s satisfying to see brains grow in size and complexity as animals evolved from swimmers to crawlers to walkers.

(Remember, though, that comparing extant species in this way only provides an indirect view of evolutionary history: modern fish have also evolved, over millions of years, from their last common ancestor with humans.)

s a species of tinkerers, we sometimes mistakenly assume that complex biological structures, like neurons, explicitly evolved to carry out brain signaling. But evolution’s only goal as an ‘tinkerer’ is to promote the survival of an existing species, not necessarily to build complex structures for future species.

Bacterial ion channels demonstrate an important lesson of modern evolutionary theory, as articulated by Richard Dawkins in his book, The Extended Phenotype, with reference to newly-evolved traits:

But the story of the human brain stretches even further back than what we can see in our animal relatives.

In fact, the story predates even the first neurons, which we often think of as the building blocks of even the simplest brains. To understand where brains come from, we have to reach back to the first examples of life forms capable of successfully reacting to their immediate environment. The evolution of the human brain begins with bacteria.

Even bacteria can think (sort of)

Obviously, single-celled organisms like bacteria utterly lack neurons, let alone brains. Modern genetics increasingly shows, however, that bacteria possess a unit of human brain structure even more microscopic than neurons: ion channels.

Ion channels are large proteins that selectively allow ions (electrically-charged molecules) to flow in and out of cells. In the human brain, as well as in the neurons of even the smallest-brained animals, ion channels are vital for communication, or signaling, between neurons. Ion channels allow for messages to travel down the lengths of individual neurons, somewhat analogous to electric charge traveling down an electrical wire. This signaling is the basis for every computation the brain carries out.

Intriguingly, many of these ion channels found in human neurons are also present in ancient organisms such as bacteria, because the genes providing the instructions for building ion channels in human neurons are also found in bacteria.

But why would bacteria need ion channels if they don’t have neurons? Some bacteria possessmechanically-sensitive ion channels, as well as microscopic propellers that drive movement, giving them a rudimentary sense of touch and the ability to move. Within a single bacterial cell, sensation, information processing (you could call it a simple form of ‘memory’), and reaction can be coordinated, partially thanks to the same ion channels responsible for human brain signaling. Eons before more complex organisms would use neurons to build nervous systems, ion channels were aiding bacteria in their quest to interact with their surroundings.

s a species of tinkerers, we sometimes mistakenly assume that complex biological structures, like neurons, explicitly evolved to carry out brain signaling. But evolution’s only goal as an ‘tinkerer’ is to promote the survival of an existing species, not necessarily to build complex structures for future species.

Bacterial ion channels demonstrate an important lesson of modern evolutionary theory, as articulated by Richard Dawkins in his book, The Extended Phenotype, with reference to newly-evolved traits:

Levi Gadye

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