We are not sure why that happened. Even when & # 39; is a topic of ongoing debate. But at some point our brains became big.
There are lots of hypotheses about how we came here, but to find supporting evidence, we need experiments on the brain of chimpanzees and humans, which involves practical (not to mention ethical) challenges. So these researchers went and built their own copies.
"It's a science fiction experiment that couldn't have happened ten years ago," says cell biologist Arnold Kriegstein of the University of California, San Francisco.
The team constructed simple, biochemically active brains from chimpanzee and human stem cells and used them to identify hundreds of genetic differences that could help explain their unique properties.
We're not just talking here or here. The researchers took cells from eight chimpanzees and ten people and used them to generate a population of 56 units, providing an unprecedented scope for accurate measurements.
Technically, the human and chimpanzee brains they developed in laboratory glass are not the fully developed clumps of wrinkled gray matter you would find inside a primate skull.
They are organoids ̵
While the line between an actual organ and its organoid derivative is unclear, it is clear that these cultures of neurological tissue cannot treat information as the real deal. But that's not the goal. There is sufficient genetic and biochemical activity in these cultures to allow for experiments that would be impossible for bona fide samples.
Extraction of DNA and proteins from the brain taken by deceased chimpanzees and humans and keeping them side by side is like comparing the final credits in two films. You can know the actors, but you lack the plots.
Brain organs allow researchers to measure how genes activate and biochemistry fluctuate, and the time of development of important cells and other structures.
Having dozens of organoids for comparison means that changes that are general to each species can be selected with precision.
The researchers deconstructed their samples at various stages of development so that they could compare the types of cells that appear and the genetic programs activated at each stage.
These were all compared to reference materials taken from a third group of primates, rhesus monkeys.
Contrasts in the genetic activity of human and chimpanzee organs provide fruitful reasons for identifying important mutations in each species that could explain how our respective brains evolved.
"These chimpanzee organs give us an otherwise inaccessible window to six million years of our development," says neurologist Alex Pollen.
The analysis revealed 261 human-specific changes in genetic expression; One particular change that got their interest was a type of neural precursor.
Several years ago, Kriegstein's laboratory identified molecular traits of a kind of cell that gives rise to the majority of human cortical neurons, called an outer radial glial cell. This time, the team showed how the activity of these cells strengthened their participation in a human growth pathway, highlighting a pivotal shift that could help explain the branching of human evolution away from our great monkey relatives.
"Being so close to game chimpanzees has made me ask questions about the development of our own species," says Pollen, who had studied the development of fish near the famous Chimpanzee research facility of Gombe Stream National Park.
"But first we had Need for genomes, stem cells and single-cell RNA sequencing to understand the evolutionary programs that drive brain development in the two species. "
Whatever the story lies behind man's extraordinarily enlarged brains and their ilk, it becomes complex. Organoids give new ways to study such activity on an unprecedented scale, which is the basis for showing how small changes in our evolutionary past have led to major differences in our biology.
This research was published in Cell Cell .