Mostly similar, yet totally different (The 1 Percent genes: Part 3)

Neocortex provides mammals, especially primates with high cognitive abilities. Chimps have a lot in common with us. And mice continue to have a strong bond even after 75 million years.

Part 1: Evolutionary Developmental Biology – A short history

Part 2: Subtle changes compounded over time

Part 4: The fascinating journey of a gene

The neocortex power-up

Paul Broca identified one of the first areas in the cortex with its function. In 1861, he discovered that a part of the frontal lobe was associated with speech, located in the primate premotor area. Upon further discoveries, what we can conclude is our brains are a product of upgrading primate brains which themselves are better versions of the mammalian ones. The neocortex appears to be a crucial upgrade for mammals since it doesn’t only add more power to process information but also provides functions to the animal. Another aspect of brain sizes is it matters what part of the brain is relatively large more than whether the size of the entire brain. It has been seen that in primates, the neocortex is about 2.3 times than what is seen in similarly sized brains of non-primates, thus supporting its stake at influencing evolution.

Visual aids are important for our understanding of the world. More so, an area in the brain called ‘ventral premotor’ plays a key role in those activities where our movements are guided by what we see, for example when you want to grasp an object. Some areas which govern crucial functions are also found to be located mostly on the left side of the brain. The parts of the brain that govern speech and language are found in the left hemisphere. The same half of the brain also controls right-handedness as well as hand gestures, another mode of communication. It is natural to wonder whether left brain, being a force behind communication, started evolving in earlier species towards its current form.

The human-chimp faceoff

Do you think that we have an advantage over other apes since we contain specialty parts in our brain? Sorry to note, but the physical differences are so small, scientists now believe that the actual upper hand may be the neural paths as well as the way neurons are stacked. Research is underway to find the crucial stages in evolution where such changes happened at the cellular level.

A comprehensive analysis of the growth rates and maturation within humans and chimps was undertaken. At an elementary level, a comparable rate of growth of skull and maturation is seen in humans is in contrast with that of a chimpanzee. Human babies are skilled at a much lower level than chimpanzees of the same age. This is despite our babies having bigger skulls and brains. The size difference does reduce with age as we grow on to have similar brain sizes, which means the maturity in human brains takes place at a slower rate. This serves as an example of how similar processes have shifted differently over time in each of these species. One fact that we need to remember always is that every function, ever biological part developed at different stages in our evolution. Our dental features are found to have formed much later, compared to our bipedal posture. Another fact, we aren’t unique to evolution, with several other animals have gone through various changes in form and function.

As noted earlier, changes happening at the genetic level should have caused us to evolve differently from apes. For comparative purposes, we will consider the genome of humans, chimpanzees, and mouse. First up, we’ll put the gene pool of humans against that of chimps. The human DNA sequence contains nearly 3 billion base pairs. Out of this, 98.8% of these base pairs are also seen in the DNA sequence of chimps. 1.2 percent variation might not sound much, but that alone amounts to 36 million base pairs, where the key differences should lay. You’ve read earlier that apes and early hominins separated from each other 6 million years ago. This translates to each of them carrying a specific half of those 36 million base pairs. So, human-specific DNA base pairs come down to 18 million.

Not all base pairs are involved in value adding functions, nor are all mutations. 5 percent of the DNA is behind coding and governing critical functions in our bodies. But here’s the absolute cracker of a fact. Every human, all of us, differ from each other at around 3 million bases. Yes, it only translates to 0.1 % of the genome, but enough to build a completely unique individual. But at the end of the day, we are all a single species. That’ the wonder of genes.

How ‘human’ are we?

Let us now analyze the differences (or similarities?) between us and mice. The earliest split between ancestors of today’s humans and mice occurred 75 million years ago. It goes without further explanation, that mice have smaller brains, but with a neocortex. And they have 99% similarity in genes with humans. Yes, you read that right. 99% of genes that make us have a corresponding match in a mouse. What does this tell you? 75 million years ago mammals started evolving. 55 million years ago primates started their separate journey. 6 million years ago hominins started their own evolution that led to us. And all these while 99 percent of the genes are organized in the same manner.

So how is it that we and mice don’t look comparable in any manner? The cause for the differences, as we now know, comes from how proteins are sequenced in these genes, with that in humans and mice varying by about 30 percent. But then again, proteins are involved mostly in physiological processes, not as much in form (we don’t look similar to mice in any way, do we?). It is believed that the genetic switches are again what pull the strings behind all the differences. Thus, much of evolution from the earliest species to mammals to primates to us, it isn’t much about controlling proteins but genes. A study conducted in the 1960s add to this theory when the scientists discovered protein sequencing is identical in chimps and humans.

Notes

This series is influenced by chapter ‘A beautiful mind: The making of Homo sapiens’ of the book Endless forms most beautiful by Sean Carroll.

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If you would like to learn more about Evolutionary Developmental Biology you can head over to this link: https://evolution.berkeley.edu/evolibrary/article/evodevo_01

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