Scientists who grew “brain organoids” from human, gorilla, and chimp cells may have just discovered why human brains grow larger. When they manipulated the previously unknown molecular switch, the primate brain tissue could grow like a human’s. Also, the reverse was possible: By manipulating the switch, a human brain could be made to grow slower, more like the brain of a gorilla or chimpanzee.
Interestingly, the researchers believe the gene switch may account for most of the size differences between human and primate brains.
From the Guardian:
“What we see is a difference in cellular behavior very, very early on that allows the human brain to grow larger,” said Dr. Madeleine Lancaster, a developmental biologist at the Medical Research Council’s Laboratory of Molecular Biology in Cambridge. “We are able to account for almost all of the size difference.”
Lancaster and the scientists used cells leftover from medical tests and operations. First, they converted the cells into stem cells. Then, they grew them in such a way that they became tiny brain organoids. Observing the growth rate, they saw human cells were proliferating more before the process to change into brain cells.
“Human brain organoids are expanded relative to nonhuman apes prior to neurogenesis,” the study states.
Early on in the process, human cells replicated far more than the primate cells before switching to become brain cells and neurons. Upon closer examination, the team found the gene switch turned on later in humans. Since the switch is turned on later, more cells are available, which convert into a larger brain.
Evolutionary Brain Regulator?
Dubbed ZEB2, the switch is an “evolutionary regulator,” the study states.
“The researchers went on to identify a gene that is crucial to the process. Known as Zeb2, it switches on later in human tissue, allowing the cells to divide more before they mature. Tests showed that delaying the effects of Zeb2 made gorilla brain tissue grow larger, while turning it on sooner in human brain organoids made them grow more like the ape ones,” wrote Ian Sample for the Guardian.
A Switch That Changes Everything?
Typically, a human brain is about three times as large as a gorilla or chimp brain, though we share 99% of our DNA with our closest relatives, Bonobos, and Chimpanzees. Scientists sequencing the chimp genome discovered how closely our DNA matched in 2005.
“This has prompted researchers to speculate whether the ancestor of humans, chimpanzees, and bonobos looked and acted more like a bonobo, a chimpanzee, or something else—and how all three species have evolved differently since the ancestor of humans split with the common ancestor of bonobos and chimps between 4 million and 7 million years ago in Africa,” wrote Ann Gibbons for Science.
When human ancestors diverged from apes in the distant past, the hominid brain rapidly expanded. Now, it appears the process may have been related to the way the ZEB2 gene is expressed. Apparently, that delayed switch makes all the difference, leading to brains three times as large as our closest relatives.