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Human, Mouse, and Fly Brains All Use the Same Basic Mechanisms

The study of brains in recent decades has yielded a very different picture from the patterns we might have expected

With differing outcomes, of course:

A new study led by researchers from King’s College London has shown that humans, mice and flies share the same fundamental genetic mechanisms that regulate the formation and function of brain areas involved in attention and movement control.

News Centre, “Humans and flies employ very similar mechanisms for brain development and function” at King’s College London (August 3, 2020)

We might have expected a gradual increase in size and complexity, corresponding with ability, leading up to the human brain. But we have learned from recent research that lemurs, with brains 1/200 the size of chimps’, pass same IQ test (the Primate Cognition Test Battery). Human intellectual abilities are orders of magnitude greater than that of any other life form, with no proportionate increase in size.

Does structure matter? Maybe. Smart birds turn out to have brains similar to those of mammals. But then smart octopuses have a quite different invertebrate brain structure. Octopuses, for example, have nine simple brains rather than one complex one.

And the human brain does not work precisely as we had come to expect either. People whose brains have been split in half (due to otherwise untreatable epilepsy) function surprisingly well, as do people with only half a brain.

Thus, it isn’t clear that the uniqueness of human intellect depends entirely on brain size, organization, or wholeness. Whatever the final picture of our brains proves to be, it will not be quite so neat as that.

The King’s College announcement also tells us,

Data has shown that the genetic mechanisms that underlie the brain development of insects and mammals are very similar but this can be interpreted in two different ways, where some believe it provides evidence of one single ancestor for both mammals and insects and others think it could support the theory that brains evolved multiple times independently.

News Centre, “Humans and flies employ very similar mechanisms for brain development and function” at King’s College London (August 3, 2020)

Origins can be a complicated business. For example, evolution can include both inheritance and convergence. According to current theory, the last common ancestor of humans, flies and birds flourished 520 million years ago. Did that ancestor really have such a complex brain? If so, that brain must have developed remarkably quickly—perhaps too quickly to be accepted as the best explanation. We can’t know for sure because brains don’t fossilize.

Convergence, on the other hand, means that life forms end up with similar attributes, not because they are inherited from a common ancestor but because they serve common needs. For example, molluscs (octopus, squid, and cuttlefish) appear to have developed nervous systems four times. They all have many limbs to control so they all evolved complex nervous systems—but not necessarily from a common ancestor; perhaps in response to a common need. Often, we don’t know the whole story so we must make a best guess.

The only certainty is that studies of the brain will continue to amaze us.

Note 1: The paper, Bridi, J. et al. (2020) Ancestral regulatory mechanisms specify conserved midbrain circuitry in arthropods and vertebrates Proceedings of the National Academy of Sciences (PNAS) DOI: 10.1073/pnas.1918797117, is open access.

Note 2: The feature photo is courtesy Tobias Roth on Unsplash. The comparative brain graphic above is courtesy of Dr. Frank Hirth of King’s College, London.

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Human, Mouse, and Fly Brains All Use the Same Basic Mechanisms