We are not sure why it happened. Even when is a topic that is constantly discussed. But someday our brain got big for some reason.
There are many hypotheses of how we got here, but to find supportive evidence, we need to experiment with the brain of chimpanzees and humans, which is practical (not) ethical challenges. So these scientists built their own specimens.
"It's a science-fiction experiment that would not have been possible ten years ago," says cell biologist Arnold Kriegstein of the University of California at San Francisco.
The team constructed simple, biochemically active brains of human stem cells from chimpanzees and from Israel and used them to identify hundreds of genetic differences that might explain their unique properties.
We're not just talking about one or two. Researchers extracted cells from eight chimpanzees and ten humans and used them to create 56 samples, providing unprecedented space for accurate measurements.
Technically, the brains of humans and chimpanzees that they have developed in laboratory glassware are not the fully developed lumpy wrinkled gray matter that would be found in the skull of a primate.
They are organoids ̵
While the line between an actual organ and its organoid derivative is out of focus, these cultures are clear information from neurological tissues can not process information such as the actual deal. But that is not the goal.
There is enough genetic and biochemical activity in these cultures to allow for experiments that would not be possible on real samples.
Extracting DNA and proteins from the brains of deceased chimpanzees and humans and keeping them side by side is like comparing the guy numbers of two films. You may know the actors, but you miss the actions.
Brain organoids enable researchers to measure how genes become active and biochemistry fluctuates, as well as the timing of the development of important cells and other structures.
Dozens of organoids comparison means that changes common to each species can be precisely worked out.
The researchers deconstructed their samples at various stages of development to compare the emerging cell types and the genetic programs activated in each step.
These were all compared to reference materials from a third group of primates, the rhesus monkeys.
Contrasts in the genetic activity of the organoids of humans and chimpanzees are a fertile reason for the identification of important mutations in each species corresponding brains evolved.
"These chimpanzee organoids give us an otherwise inaccessible window for six million years of evolution," says neurologist Alex Pollen.
The analysis revealed 261 human-specific changes in genetic expression; One particular change that aroused their interest was a kind of neural precursor.
A few years ago, Kriegstein's laboratory identified the molecular characteristics of a cell type that produces most human cortical neurons, termed outer radial glial cells. This time around, the team showed how the activity in these cells increased participation in a human growth path, and pointed to a significant shift that could explain the diversion of human evolution from our great ape relatives.
"Being so close to wild chimpanzees made me wonder about the evolution of our own species," says Pollen, who studied the development of fish near the famous chimpanzee research facility at Gombe Stream National Park.
"But first we needed genomes, stem cells, and single-cell RNA sequencing to understand the evolutionary programs that drive brain development in the two species."
Whatever the story behind It is becoming more complex to the unusually enlarged brains of man and his ilk. Organoids provide new ways to study such activities at unprecedented levels, and lay the groundwork for showing how small changes in our evolutionary past have led to large differences in our biology.
This research was published in Cell .