Primates are difficult to define unambiguously based on morphological attributes.
Most primates have hands and feet adapted for grasping.
Compared to other mammals, primates have larger brains and shorter jaws.
Primates have flat nails on their digits instead of narrow claws.
They exhibit well-developed parental care and complex social behavior.
The earliest primates likely lived in trees, shaped by natural selection for arboreal life.
Grasping hands and feet are adaptations for holding onto tree branches.
All modern primates, except Homo, have a big toe widely separated from other toes.
The thumb is mobile and separate from fingers in all primates, but a fully opposable thumb is only found in anthropoid primates.
Human dexterity, supported by distinctive bone structure at the thumb base, evolved from ancestral hands adapted for tree life.
Overlapping fields of vision in the two eyes enhance depth perception, advantageous for brachiating.
Excellent hand-eye coordination is crucial for arboreal maneuvering.
Primates are divided into two subgroups:
Prosimii (prosimians): Resemble early arboreal primates, including lemurs, lorises, pottos, and tarsiers.
Anthropoidea (anthropoids): Include monkeys, apes, and humans.
The oldest known anthropoid fossils, from about 45 million years ago, suggest tarsiers are the prosimians most closely related to anthropoids.
By about 40 million years ago, monkeys existed in Africa, Asia, and South America.
Old World and New World monkeys underwent separate adaptive radiations.
New World monkeys are all arboreal and have prehensile tails and nostrils that open to the side.
Old World monkeys include both arboreal and ground-dwelling species, lack prehensile tails, and have nostrils that open downward.
The anthropoid suborder includes four genera of apes:
Hylobates (gibbons)
Pongo (orangutans)
Gorilla (gorillas)
Pan (chimpanzees and bonobos).
Modern apes are confined to tropical regions of the Old World and evolved from Old World monkeys about 25-30 million years ago.
Except for gibbons, modern apes are larger than monkeys, with long arms, short legs, and no tails.
While all apes can brachiate, only gibbons and orangutans are primarily arboreal.
Social organization varies, with gorillas and chimpanzees being highly social.
Apes have larger brains than monkeys, and their behavior is more flexible.
Hominins Versus Hominids
Humans and apes share ancestry for most of life's history, except for the last few million years.
Paleoanthropology is the study of human origins and evolution, focusing on the period when humans and chimpanzees diverged from a common ancestor.
Hominid: Refers to great apes and humans collectively (Family Hominidae).
Hominin: Specifically refers to the evolutionary line more closely related to humans than to any other living species.
There are two main groups of hominins:
Australopithecines (all extinct)
Homo (with all species extinct except Homo sapiens).
Human ancestors were not chimpanzees or any other modern apes.
Chimpanzees and humans evolved from a common ancestor that was neither a chimpanzee nor a human.
Human evolution did not occur as a ladder-like progression to Homo sapiens.
Human phylogeny is more like a multibranched bush with our species as the tip of the only surviving twig.
Evolutionary Tree for Hominins
Upright posture and enlarged brains did not evolve in unison; different features evolved at different rates (mosaic evolution).
Early ancestors walked upright but had less developed brains.
Humanity's anthropoid ancestors of 30-35 million years ago were tree dwellers.
Around 20 million years ago, climate change led to the contraction of forests and expansion of savanna habitats in Africa and Asia.
Major evolutionary changes occurred as ancestors spent less time in trees and more time on the ground.
Fossil records and DNA comparisons indicate that humans and chimpanzees diverged from a common hominoid ancestor about 5-7 million years ago.
The Molecular Clock
The molecular clock suggests that molecular differences between species are proportional to the time of separation, not the degree of phenotypic difference.
Measures of genetic divergence can be used to date the time of divergence for species pairs without fossil data.
Strong Linear relationship suggests that molecular differences between pairs of species are proportional to the time of their separation, not the degree of phenotypic difference.
Measures of genetic divergence can be used to date the time of divergence for species pairs for which no fossil data are available
Species that appear to be physically similar to humans have more similar DNA sequences for the same genes when compared to species that are less physically similar
Brain size in hominoids tripled over the past 6 million years, increasing from 400-450 cm3 to about 1,300 cm3 in modern humans.
Hominin ancestors had longer, prognathic jaws, leading to flatter faces with more pronounced chins in modern humans.
Bipedal Posture
Hominid ancestors walked on all four limbs on the ground, like modern apes.
Bipedal posture is associated with key skeletal modifications in early hominin fossils.
Hypothesized causes of bipedalism include:
Energy-efficient locomotion.
Provisioning offspring.
Food gathering.
Freeing the hands.
Predator avoidance.
Tracking migrating herds.
An upright stance reduces heat absorption at midday by 60% compared to a quadrupedal stance.
An aquatic ape theory was referenced
Family Structure and Sexual Dimorphism
Size differences between sexes, a major feature of sexual dimorphism.
Male gorillas and orangutans are about twice as heavy as females.
Male chimpanzees and bonobos are about 1.35 times heavier than females.
Human males average about 1.2 times the weight of females.
Monogamy and long-term pair-bonding prevail in most human cultures, unlike most ape species.
Human infants are exceptionally dependent, requiring extended parental care.
Opportunities for enhanced learning are much longer in humans than in other hominoids.
The Fossil Evidence
All known hominin fossils older than 1.5 million years are from eastern and southern Africa.
Fossils mainly consist of teeth and fragments of jaws, skulls, and other skeletal pieces.
Researchers reconstruct human phylogeny from incomplete records, revising hypotheses with new fossil evidence and molecular systematics data.
The various pre-Homo hominids are classified in the genus Australopithecus.
Australopithecus africanus was discovered in 1924 by Raymond Dart in South Africa.
A. africanus likely walked fully erect and had humanlike hands and teeth, but a brain only about one-third the size of a modern human's brain.