The evolution of the human brain is a fascinating subject. The brain is arguably the most complex structure in the known (to us) universe, and is the feature that makes humanity unique and has allowed us to dominate (for good or ill) the fate of this planet. But of course we are but a twig on a vast evolutionary tree, replete with complex brains. From a human-centric perspective, the closer groups are to humans evolutionarily, the more complex their brains (generally speaking). Apes are the most “encephalized” among primates, as are the primates among mammals, and the mammals among vertebrates. This makes evolutionary sense – that the biggest and most complex brains would evolve from the group with the biggest and most complex brains.

But this evolutionary perspective can be tricky. We can’t confuse looking back through evolutionary time with looking across the landscape of extant species. Any species alive today has just as much evolutionary history behind them as humans. Their brains did not stop evolving once their branch split off from the one that lead to humans. There are therefore some groups which have complex brains because they are evolutionarily close to humans, and their brains have a lot of homology with humans. But there are also other groups that have complex brains because they evolved them completely independently, after their group split from ours. Cetaceans such as whales and dolphins come to mind. They have big brains, but their brains are organized somewhat differently from primates.

Another group that is often considered to be highly intelligent, independent from primates, is birds. Birds are still vertebrates, and in fact they are amniotes, the group that contains reptiles, birds, and mammals. It is still an open question as to exactly how much of the human brain architecture was present at the last common ancestor of all amniotes (and is therefore homologous) and how much evolved later independently. To explore this question we need to look at not only the anatomy of brains and the networks within them, but brain cell types and their genetic origins. For example, even structures that currently look very different can retain evidence of common ancestry if they are built with the same genes. Or – structures that look similar may be built with different genes, and are therefore evolutionarily independent, or analogous.

With that background, we now have a publication of several research projects examining the brain of various amniotes – Evolutionary convergence of sensory circuits in the pallium of amniotes. The pallium is basically the cerebral cortex – the layers of gray and white matter that sit on top of the cerebrum. This is the “advanced” part of the brain in vertebrates, which include the neocortex in humans. When comparing the  pallium of reptiles, birds, and mammals, what did they find?

 “Their neurons are born in different locations and developmental times in each species,” explains Dr. García-Moreno, head of the Brain Development and Evolution laboratory, “indicating that they are not comparable neurons derived from a common ancestor.”

Time and location during development is a big clue as to the evolutionary source of different cells and structure. Genes are another way to determine evolutionary source, so a separate analysis looked at the genes that are activated when forming the pallium of these different groups. It turns out – they use very different assemblages of genes in developing the neurons of the pallium. All this strongly suggests that extant reptiles, birds, and mammals evolved similar brain structures independently after they split apart as groups. They use different neuron type derived from different genes, which means those neurons evolved from different ancestor cell types.

To do this analysis they looked at hundreds of genes and cell types across species, creating an atlas of brain cells, and then did (of course) a computer analysis:

“We were able to describe the hundreds of genes that each type of neuron uses in these brains, cell by cell, and compare them with bioinformatics tools.” The results show that birds have retained most inhibitory neurons present in all other vertebrates for hundreds of millions of years. However, their excitatory neurons, responsible for transmitting information in the pallium, have evolved in a unique way. Only a few neuronal types in the avian brain were identified with genetic profiles similar to those found in mammals, such as the claustrum and the hippocampus, suggesting that some neurons are very ancient and shared across species. “However, most excitatory neurons have evolved in new and different ways in each species,” details Dr. García-Moreno.

Convergent evolution like this occurs because nature finds similar solutions to the same problem. But if they evolved independently, the tiny details (like the genes they are built from) will differ. But also, a similar solution is not an identical solution. This means that bird brains are likely to be different in important ways from mammalian brains. They have a different type of intelligence that mammals, primates, and humans do (just like dolphins have a different type of intelligence).

This is the aspect of this research that fascinates me the most – how is our view of reality affected by the quirky of our neurological evolution? Our view of reality is mostly a constructed neurological illusion (albeit a useful illusion). It is probable that chimpanzees see the world in a very similar way to humans, as their brains diverged only recently from our own. But the reality that dolphin or crow brains construct might be vastly different than our own.

There are “intelligent” creatures on Earth that diverge even more from the human model. Octopuses have a doughnut shaped brain that wraps around their esophagus, with many of the neurons also distributed in their tentacles. They have as many neurons as a dog, but they are far more distributed. Their tentacles have some capacity for independent neurological function (if you want to call that “thought”). It is highly likely that the experience of reality of an octopus is extremely different than any mammal.

This line of thinking always leads me to ponder – what might the intelligence of an alien species be like? In science fiction it is a common story-telling contrivance that aliens are remarkably humanoid, not just in their body plan but in their intelligence. They mostly have not only human-level intelligence, but a recognizably human type of intelligence. I think it is far more likely that any alien intelligence, even one capable of technology, would be different from human intelligence in ways difficult (and perhaps impossible) for us to contemplate.

There are some sci fi stories that explore this idea, like Arrival, and I usually find them very good. But still I think fiction is just scratching the surface of this idea. I understand why this is – it’s hard to tell a story with aliens when we cannot even interface with them intellectually – unless that fact is part of the story itself. But still, there is a lot of space to explore aliens that are human enough to have a meaningful interaction, but different enough to feel neurologically alien. There are likely some constants to hold onto, such as pleasure and pain, and self-preservation. But even exploring that idea – what would be the constants, and what can vary, is fascinating.

This all relates to another idea I try to emphasize whenever relevant – we are our neurology. Our identity and experience is the firing of patterns of neurons in our brains, and it is a uniquely constructed experience.