Why do we have big heads?
The model showed why the human brain is larger than that of monkeys
Maria Krivochenko, Naked Science
At birth, there are about three times more neurons in the human brain than in newborn chimpanzees and gorillas, although we and primates have similar gestation periods. To understand why this happened, scientists from the MRC Molecular Biology Laboratory in Cambridge, UK, have grown miniature brain organoids. So they imitated the development of the organs of monkeys and humans.
Details of the work are published in the journal Cell (Benito-Kwiecinski et al., An early cell shape transition drives evolutionary expansion of the human forebrain).
To begin with, scientists collected gorilla, chimpanzee and human neurons and genetically reprogrammed them so that they resembled embryo cells, that is, induced pluripotent stem cells (iPSCs). "We kind of tricked them into thinking they were embryonic again," explained Madeline Lancaster, the head of the study.
The team then grew organoids using iPSCs. Like the present, the human brain model grew larger than that of gorillas and chimpanzees in two days. By five weeks, the organoids of the human brain were about twice as large, measuring about four millimeters across.
At the age of five weeks, the organoids of the human brain (left) are much larger than those of a gorilla (top right) and a chimpanzee (bottom right). A drawing from an article in Cell.
"The early stage of development is usually inaccessible," Lancaster said. "It's something like a black box in human biology. In addition, we know very little about this stage in gorillas and chimpanzees. Usually we don't even know if a gorilla is pregnant that early."
The researchers then analyzed the genes in the brain organoids and found differences in the expression of the ZEB2 gene. Gorilla and chimpanzee organoids included it earlier than human ones. It turned out that ZEB2 controls the shape and mobility of the precursor cells of neurons. In fact, when it is turned on, the precursors stop reproducing earlier. Human cells turn on this gene later and stay in the precursor state for longer. Due to this, during the development of the embryo, neurons become larger – and the size of the brain increases.
To make sure that ZEB2 is responsible for brain growth, the team turned it on earlier in human organoids, and later in gorilla organoids. As a result, human cells became similar to monkey cells – and vice versa. The scientists noted that the grown organoids are only a model, so they do not imitate the work of a real brain. However, they are enough to explore the key stages of the development of this organ.
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