First Primate Review

How did primates – the group of mammals that includes us humans – evolve? Comparative analysis of the genetic and fossil characteristics of primates and their ancestors is now providing new clues. Researchers have discovered that the original primate genome contains far more genes for digesting high-fat foods than other closely related mammals. They take this as an indication of adaptation to meat-heavy foods – eg in the form of insects. In parallel, the first primates developed forward-facing eyes and grasping hands, which made it easier for them to climb trees and see in three dimensions. By comparing them to other carnivores, scientists have concluded that the first primates may have been arboreal ambush predators.

Lemurs, monkeys, apes and humans are monkeys. Typical of this arrangement of mammals are large, forward-facing eyes, relatively large brains, hands that are adapted for grasping, and flat nails rather than claws on fingers and toes. Fossil finds indicate that the first representatives of primates existed already at the end of the Cretaceous period about 66 million years ago. How and why these first primitive primates developed their own characteristics has been a matter of debate until now. It seems clear that holding hands made life in trees easier, and placing the eyes in front of the head, which is convenient for three-dimensional vision, would have also made it easier for primitive primates to move in the branches. . However, it is unclear what role the nocturnal lifestyle played for large, parallel eyes and how they fed the first primates: were insectivorous animals like some of the smaller representatives of this group today? Or were their flexible hands and fingers more adapted to fruits and nuts?

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Digestive genes reveal diet

“There is evidence supporting both hypotheses, but which one is correct can only be clarified by reconstructing the diet of the original animals,” says Yonghua Wu of Northeast Normal University in China and colleagues. To get more clarity on this, they first looked for genetic evidence in 32 primate species and four closely related mammals as comparison groups. They compared the expression and development of 117 genes associated with digestion and the enzymes required for it. “These genes play an important role in the digestion and absorption of carbohydrates, proteins and fats,” the scientists explained. Since a vegetarian diet is rich in carbohydrates, while a diet rich in meat contains more fats and proteins, the food preferences of different primates should also be reflected in these genes.

As the analyzes showed, digestive genes in primates actually show a developmental trend: Compared with control groups and also within different primate lineages, the number of genes responsible for fat digestion increased significantly, the team found. However, this trend was not evident in the genes for other nutrients. According to the researchers, this indicates that the first primates gradually evolved from omnivores to a more meat-dense diet. While closely related mammals still used genes to digest carbohydrates, proteins, and fats, early primates had an increased proportion of fat-digesting genes. “This positive selection of genes for fat digestion suggests that the original euprimates had a high-fat diet,” Wu and his team report. “Maybe they were carnivores.” This corresponds to the fossils of early primates, whose teeth indicate a diet dominated by insects.

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Eyes and hands adapted to the ambush?

Based on this finding, Wu and colleagues investigated to what extent this diet combined with an arboreal lifestyle could also explain other traits of early apes, such as their graspable hands, lowered claws, and front eyes. To do this, they compared the position of the eye of primates with that of other carnivorous vertebrates such as owls, cats, wolves, or eagles. “Among these predators, those with more forward-facing eyes, such as owls and cats, typically appear to be ambush hunters, while others are more likely to be stalkers,” the team says. Usually ambush hunters wait as much as possible for the prey to approach, then rush to capture and kill it. However, this only works if they can surprise prey and if their attack is as targeted and accurate as possible.

According to Lu and his team, early primates may also have relied on this element of surprise when hunting insects. “The cover-up may have opened a new field for the original Euprimates that were beyond the reach of their relatives,” the researchers explain. Adaptations to this type of hunting bugs could also explain why primates develop good three-dimensional vision and accurate, mobile grasping fingers: “Good three-dimensional vision and thus parallel eyes can be crucial, especially for ambush hunters, to determine the distance to the animal.” to accurately assess prey. During their sudden attack,” the scientists explain. Holding hands provided a good grip when jumping and grabbing prey. At the same time, their claws might have become increasingly smaller and flatter: “Soft pads of fingers and feet without claws reduce noise when climbing,” Wu and colleagues write. General, you see some indication that early primates evolved at least some of their typical characteristics in adapting to insect ambushes in primitive treetops.

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Source: Yonghua Wu (Northeast Normal University, Changchun) et al., Science Advances, doi: 10.1126/sciadv.abn6248

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