Earliest Primates

Primates evolved more than 60 million years ago during the Paleocene. The first primates resembled tree shrews with long snouts and poor daytime vision. "Uniquely" human traits evolved through modification of traits that evolved earlier, in ancestral forms.

Primates include:

Prosimians literally means before apes. They are small tree dwellers that use their large eyes to advantage during night hunting.

Tarsioids (tarsiers) are small primates with features intermediate between prosimians and anthropoids.

Anthropoids include monkeys, apes , and humans.

Hominoids include apes and humans and extinct species of their lineages. In biochemistry and body form, humans are closer to apes than to monkeys. Hominids are a subgroup that includes humans and extinct humanlike species. Hominid refers to human lineages only.

Most primates live in tropical or subtropical forests, woodlands, or savannas. With the exception of humans, most primates are tree dwellers.

No one feature distinguishes the primates from other mammals, but there are several trends that define the lineage:

There was less reliance on sense of smell and more on daytime vision.
Skeletal changes led to upright walking (bipedalism), which freed the hands for novel tasks.
Changes in bones and muscles led to refined hand movements.
Teeth became smaller and less specialized. The jaw became bow-shaped.
Changes in the brain and behavior became interlocked with each other and with cultural evolution.
Longer lifespan and longer period of dependency by the young.

The first hominoids appeared about 20 million years ago during the Miocene. Continents were settling into their current positions, climates were cooler and drier, and forests gave way to grasslands.

The first hominoids ranged over the Old World and became extinct except for some of their descendants, which branched three ways into gorillas, chimps, and eventually, humans.

The First Hominids

Most of the earliest known hominids lived in the East African Rift Valley. Cooler and drier weather encouraged the transition of hominids to mixed woodlands and grasslands. The early hominids had these features in common:

They were upright walkers, with hands freed for new tasks.
Modifications in the teeth and jaws allowed a more varied diet.
The more elaborate brain permitted them to think ahead and reason.

The earliest known hominid is Ardipithecus ramidus who lived 4.4 million years ago in Africa. It was more apelike than humanlike.

Numerous australopiths (southern apes) evolved during the next 2 million years. They had a large face, protruding jaw, small skull, and walked upright.

Gracile (slightly built) forms have been designated Australopithecus anarnensis, A. afarensis and A. africanus.

Robust forms that arose later and had heavier builds are designated A. boisei and A. robustus. The exact family tree is not known.

Hominids began to use stone tools about 2.5 million years ago to get marrow out of bone and to scrape flesh from bones.

The first member of the genus Homo and the first toolmaker is referred to as "handy man" or Homo habilis and lived in woodlands.

Early Homo had a smaller face, more generalized teeth, and larger brain. "Manufactured" tools have been found at Olduvai Gorge in Africa.

Africa appears to be the cradle of human evolution. No human fossils older than 1.8 million years have been found anywhere except Africa. Homo erectus left Africa in waves from 2 million to 500,000 years ago.

Where modern human populations arose is a hotly debated topic. The African emergence model says that modern human populations evolved in Africa and then migrated to the other parts of the world. The multiregional model says that H. erectus migrated and then subdivided by genetic divergence. Both models attempt to address both biochemical and fossil evidence.

The multiregional model argues that H. erectus migrated to many locations by about 1 million years ago. Geographically separated populations gave rise to phenotypically different races of H. sapiens in different locations. Gene flow prevented races from becoming species.

The African emergence model argues that H. sapiens arose in sub-Saharan Africa. H. sapiens migrated out of Africa and into regions where H. erectus had preceded them. Only after leaving Africa did phenotypic differences between races arise.

Homo erectus had a larger brain than H. habilis, had a long, chinless face, thick-walled skull, heavy browridge but was narrow-hipped and long-legged. Homo erectus made advanced stone tools and used fire as they migrated out of Africa into Asia and Europe.

Neanderthals were similar to modern humans but disappeared 35,00 - 40,000 years ago. These early humans lived in Europe and the Near East. They were massively built, with large brains and disappeared when H. sapiens appeared. DNA evidence suggests that they did not contribute to modern European populations.

REVIEW: Various mammals _____ .
a. hatch
b. finish embryonic development in a pouch
c. finish embryonic development in a uterus
d. both b and c
e. all of the above