How did primates evolve—the group of mammals that includes us humans? A comparative analysis of genetic and fossil characteristics of primates and their ancestors now provides new evidence. Researchers have discovered that the genome of original primates contains significantly more genes for digesting high-fat food than that of other closely related mammals. They take this as an indication of an adaptation to meat-heavy food - for example in the form of insects. In parallel, the first primates evolved forward-facing eyes and grasping hands, which made it easier for them to climb trees and see in three dimensions. By comparing them with other carnivorous animals, the scientists conclude that the first primates were probably arboreal ambush predators.
Lemurs, monkeys, apes and humans are primates. Typical of this order of mammals are large, forward-facing eyes, relatively large brains, hands adapted for gripping, and flat nails rather than claws on the fingers and toes. Fossil finds suggest that the first representatives of primates already existed at the end of the Cretaceous period around 66 million years ago. How and why these first primeval primates developed their special characteristics has so far been disputed. It seems clear that the gripping hands made life in trees easier, and the position of the eyes in front of the head, which is favorable for three-dimensional vision, could also have made it easier for the primeval primates to move around in the branches. However, it is unclear what role a nocturnal lifestyle played for the large, parallel eyes and how the first primates fed: Were they insectivores like some smaller representatives of this group today? Or were their flexible hands and fingers more adapted to grasping fruit and nuts?
Digestive genes reveal diet
"There is supporting evidence for both hypotheses, but which one is correct can only be clarified by reconstructing the diet of original euprimates," say Yonghua Wu of Northeast Normal University in China and his colleagues. To get more clarity on this, they first looked for genetic clues in 32 primate species and four closely related mammals as comparison groups. They compared the expression and development of 117 genes that are linked to digestion and the enzymes required for it. "These genes are known to play an important role in the digestion and absorption of carbohydrates, proteins and fats," the scientists explain. Because a plant-based diet is rich in carbohydrates, while a meat-heavy diet contains more fat and protein, the preferred food of the various primates should also be reflected in these genes.
As the analyzes showed, the digestive genes in the primates actually show a development trend: Compared to the control groups and also within the different primate lines, the number of genes responsible for fat digestion increased significantly, as the team found. However, this trend was not evident in the genes for other nutrients. According to the researchers, this suggests that the first primates gradually evolved from omnivores to a more meat-heavy diet. While the closely related mammals still used genes for the digestion of carbohydrates, proteins and fats, the proportion of fat digestion genes increased in the early primates. "This positive selection of genes for fat digestion suggests that the original euprimates had a high-fat diet," Wu and his team report. "They were probably carnivores." This is consistent with fossils of early primates, whose teeth indicate a predominantly insect-based diet.
Eyes and hands adapted to ambush?
Based on this result, Wu and his colleagues investigated to what extent this diet combined with the arboreal lifestyle can also explain other features of early primates, such as their prehensile hands, reduced claws and forward-facing eyes. To do this, they compared the primates' eye position 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 the others are more likely to be chasers," the team reports. Ambush hunters typically wait as hidden as possible for prey to approach, then lunge to catch and kill it. However, this only works if they can surprise the prey and if their attack is targeted and as accurate as possible.
According to Wu and his team, early primates may have also relied on this element of surprise when hunting insects. "Lurking may have opened up a new niche for the original Euprimates that was inaccessible to their relatives," the researchers explain. Adaptation to this type of insect hunting could also explain why the primates developed good three-dimensional vision and mobile, precise gripping fingers: "Good three-dimensional vision and thus parallel eyes could be crucial, especially for ambush hunters, to determine the distance to the animal." to precisely assess prey during their surprise attack," the scientists explain. The gripping hands provided a good grip when jumping and grabbing the prey. At the same time, their claws may have become increasingly reduced and flatter: "Soft finger and toe pads without claws reduce the noise when climbing," write Wu and his colleagues. All in all, she sees some indications that the early primates developed at least some of their typical characteristics in adaptation to ambushing insects in the primeval treetops.
Source: Yonghua Wu (Northeast Normal University, Changchun) et al., Science Advances, doi: 10.1126/sciadv.abn6248