What makes humans different should be straightforward, right? We should, for example, have more complex neurons than ferrets and macaques.

But we don’t. We have simpler ones:

Neurons communicate with each other via electrical impulses, which are produced by ion channels that control the flow of ions such as potassium and sodium. In a surprising new finding, MIT neuroscientists have shown that human neurons have a much smaller number of these channels than expected, compared to the neurons of other mammals.

MIT, “A Striking Difference Between Neurons of Humans and Other Mammals” at Neuroscience News (November 10, 2021) The paper requires a subscription.

In the most extensive study of its kind, nine other mammals were studied. Larger mammals have larger neurons. And in every case but one, they found that “as the size of neurons increases, the density of channels found in the neurons also increases.” Except in humans:

“Previous comparative studies established that the human brain is built like other mammalian brains, so we were surprised to find strong evidence that human neurons are special,” says former MIT graduate student Lou Beaulieu-Laroche.

MIT, “A Striking Difference Between Neurons of Humans and Other Mammals” at Neuroscience News (November 10, 2021) The paper requires a subscription.

Scrambling for an explanation of the unexpected finding, the researchers offer:

“If the brain can save energy by reducing the density of ion channels, it can spend that energy on other neuronal or circuit processes,” says Mark Harnett, an associate professor of brain and cognitive sciences, a member of MIT’s McGovern Institute for Brain Research, and the senior author of the study …

In this case, the human brain could run on the same amount of power as other mammal brains but perform more complex procedures with the excess energy diverted from ion channels.

Deborah Davis, “How efficient is the human brain?” at Cosmos (November 11, 2021)

Two proposed areas of future research are 1) where the energy not required for ion channels in humans is actually going and 2) whether genes found only in humans are responsible for the reduced ion channel density.

Another recent (and unexpected) finding is that humans use what was formerly called junk DNA (the 98% of DNA that does not code for proteins) differently from chimpanzees. The difference, researchers report, plays a role in the development of the human brain:

Researchers grew brain cells from humans and chimpanzees using stem cells and compared the two cell types, finding that they use the non-coded part of their DNA in different ways, which appears to play a considerable role in the development of human brains.

Staff, “Overlooked DNA may be what separates humans from other primates – study” at Jerusalem Post (November 14, 2021) The paper is open access.

In the past, scientists were looking for differences between humans and chimpanzees in coding DNA only:

“Previously, researchers have looked for answers in the part of the DNA where the protein-producing genes are – which only makes up about 2% of our entire DNA – and examined the proteins themselves to find examples of differences,” explained Jakobsson. “Our results indicate that what has been significant for the brain’s development is instead perhaps hidden in the overlooked 98%.”

Staff, “Overlooked DNA may be what separates humans from other primates – study” at Jerusalem Post (November 14, 2021) The paper is open access.

The researchers don’t yet know what difference the different non-coding DNA makes. But it certainly sheds an interesting light on claims that physical differences between humans and chimpanzees are negligible (“the 98”). That clearly depends on what one chooses to count.

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