In the hope of understanding how humans developed their large brains, researchers have turned to our primate relatives. A new study suggests that primates and humans have bigger brains as a result of the search for food rather than because they developed in social societies. These results call into question'the social brain hypothesis,' which suggested that humans are big-brained due to factors associated with sociality. The researchers examined more than 140 species of primate. They found that species that exist solely on fruit or a mixture of fruit and leaves were found to have larger larger brains than exclusively leaf eaters.
WASHINGTON – Scientists have long been baffled by the smarts displayed by some birds with tiny brains. But a new explanation may turn the term "bird brain" on its head: Birds have more densely packed neurons in their brains than other animals, contributing to cognitive ability on par with that of primates, researchers said on Monday. A macaw's brain may be the size of a shelled walnut, far smaller than that of a macaque monkey -- which has a brain the size of a lemon -- but the parrot has many more neurons, or brain nerve cells, in its forebrain, a region crucial for intelligence, according to a study published in the Proceedings of the National Academy of Sciences. The researchers were the first to systematically measure neurons in the brains of 20 bird species ranging in size from the tiny finch to the six-foot (1.8-meter) emu. "For a long time having a'bird brain' was considered to be a bad thing," said senior author Suzana Herculano-Houzel, a neuroscientist at Vanderbilt University.
Primates have larger and more complex brains than those of reptiles. The cortices of reptile and bird brains are formed through direct neurogenesis, as radial glia divide to generate neurons. In primates, on the other hand, an amplification step is thrown in with the intermediate progenitors that results in more neurons. Cárdenas et al. look at the patterns and regulators of neurogenesis in snake, chicken, mouse, and human brain organoids. Experimental manipulations that directed more signaling from Roundabout (Robo) transmembrane receptors and less from the Notch ligand Dll1 caused human brain organoids to lose indirect neurogenesis, whereas less Robo and more Dll1 caused snake embryos to gain indirect neurogenesis.