Final answer:
The four primate evolutionary grades are Prosimians, Ceboids, Hominidae (Great Apes), and Hominins, each showing progressive developments in brain size, arboreal adaptations, and social behaviors.
Step-by-step explanation:
Four Primate Evolutionary Grades
Primates have evolved into several stages or grades of complexity over time. The evolutionary grades in the order of primates include the following:
- Prosimians: This is the most ancient group of primates which includes species such as lemurs, aye-ayes, lorises, and tarsiers. They tend to be nocturnal and smaller in size with relatively smaller brains compared to anthropoids.
- Ceboids: These are the monkeys of South America, such as marmosets and howler monkeys. They are characterized by their ability to hang from branches with their tails.
- Hominidae (Great Apes): This group includes the four living genera of chimpanzees, gorillas, orangutans, and humans, which are known for their larger brains and more complex behaviors.
- Hominins: This group is within the family Hominidae and includes genera that are closely related to humans. These evolved about 2.8 million years ago and were fully bipedal with small brains compared to modern humans.
All of these groups exhibit characteristic primate features such as pentadactyl limbs, various tooth types, stereo vision, and opposable thumbs.