In 1859, Darwin published
The Origin of the Species, which has become the basis for the modern
day theory of evolution. From his studies, particularly those he
conducted in the Galopagos Islands, Darwin was convinced that animal
species are mutable or capable of change. At the time, it was already
- Not all individuals that are born
will reproduce. If they did, populations of animals would never
stop increasing, but populations always level off, roughly.
- There are individual differences
- Many individual differences are
heritable, which means that offspring are likely to resemble
Darwin was responsible
for hypothesizing that:
- Certain characteristics are more
likely to ensure an individual survival so individuals with these
characteristics are more likely to reproduce. This is known as
- Some characteristics make an individual
more likely to procure a mate. The ability to compete with other
members of the same sex or the possession of certain characteristics
that make an individual more attractive to the opposite sex help
to ensure that an individual will reproduce. This is known as
sexual selection. Characteristics such as the colorful plumage
of males of many bird species are believed to have originated
from sexual selection.
- From all of this, Darwin concluded
that all species change gradually over time.
These are the basic
premises of Darwin's theory of evolution. He believed that this explained
the diverse array of species that exist today. He concluded that
fossil remains of extinct species were evidence of a continuum of
characteristics shared with modern day (extant) species. His theory
also provides an explanation for the resemblance among the various
species of primates, particularly the Great Apes. Darwin posited
that humans were a twig on the tree of primate evolution, sharing
a common ancestor with chimpanzees. Both fossil evidence and studies
of living primates have offered us insight as to how primates might
have evolved into a distinct Order of mammals.
adaptations have made it possible for primates to inhabit diverse
ecological areas. An example of an extremely significant adaptation
in primate species is the development of prehensile hands and feet
capable of fine and precise motor control. Thus primates can easily
move food from hand-to-mouth, and perform gymnastic locomotor behaviors
such as leaping and jumping from branch to branch among tall trees.
Adaptations of the mammalian skeleton included a clavicle, or collar
bone, for increased movement in the shoulder; two separated bones
in the arms and legs (ulna and radius and tibia and fibula); five
digit hands and feet; one or more nails instead of claws; sensitive
touch pads on hands and feet; and fewer kinds of teeth for obtaining
and processing food. Primates developed with an increased emphasis
on vision thus overlapping, or stereoscopic, color vision became
more important as a means of gathering information from the environment
. Therefore, eyes are located on the front of primate skulls. These
biological changes were necessary for the development of depth perception
of arboreal dwellers and their quick movement through tall trees
and dense vegetation. The emphasis on vision means that the importance
of smell was reduced. The diminished dependence on smell was reflected
in the reduced snout of primates.
In time, the ratio
of the primate brain grew larger in relation to body size and more
complex behavior resulted from the addition of neurological tissue.
Primate behavior became increasingly more complex, enabling them
to live in larger social groups and inhabit a greater range of ecological
niches. The complexity of primate behavior is an important factor
in the success and proliferation of primate species to other continents.
Primates are heavily
dependant on learned behavior. Information must be learned and shared
through social groups composed of infants, children, and adults.
All stages of growth and development, as well as adulthood were lengthened
to accommodate the acquisition and passage of information from one
generation to the next generation. Complex social behaviors are learned
from play, trial-and-error, problem solving and observational learning.
4,500 - 2,500 mya (million years ago)
The earth begins to form as dust particles condense into a hot, molten
The first known rocks form as the earth cools. The atmosphere is
forming as gas escapes the earth's hardening crust. There is no free
oxygen (O2). The cooling of the earth causes water vapor to condense
and flood the earth, creating the oceans.
The earliest known life begins. These single-celled organisms, prokaryotes,
have no nucleus and resemble present day bacteria.
The food source of the prokaryotes, the organic molecules of the
oceans, has depleted. Prokaryotes that are self-feeding, autotrophic,
have an advantage in survival and flourish. Prokaryotes develop photosynthesis,
a process by which they combine carbon dioxide and water with energy
from the sun to form simple sugars. As a byproduct of photosynthesis,
free oxygen collects in the atmosphere.
2,500 - 570 mya
2,000 mya: Prokaryotes branch out into many types of single-celled
1,200 mya: Eukaryotes, the first organism with a
nucleus, may have originated, though this may not have occurred until
later. Divergence of the plant and animal phyla.
1,000 mya: The evolution of sexual reproduction.
This creates more diversity in organisms.
750 mya: Evolution of the first multicellular plants
and animals (invertebrates).
570 - 225 mya
570 mya: Cambrian
period begins. First known arthropods, shelled ancestors of present-day
insects. A wide variety of invertebrate species develops.
500 mya: Ordovician period begins. The first vertebrates,
fish, evolve. Shell bearing invertebrates flourish.
435 mya: Silurian period begins.The first dry land
plants develop. Arthropods become terrestrial. Armored fish flourish
in the oceans.
395 mya: Devonian period begins. Forests of giant
ferns are emerging on the land. Arthropods evolve into insects. Vertebrate
fish evolve into amphibians and begin utilizing resources on the
345 mya: Carboniferous period begins.The surface
of the earth is becoming dryer. Some amphibians evolve into reptiles.The
first seed-bearing, or gymnosperm, plants and coniferous trees develop.
280 mya: Permian period begins. Forests of gymnosperm
plants are replacing the primitive giant ferns. The first fossil
record of the Dimetrodon, a mammal-like reptile which later evolves
into the order Mammalia.
225 - 65 mya
225 mya: Triassic
period begins. Spread of mammal-like reptiles.The earliest dinosaurs
180 mya: Jurassic period begins. Dinosaurs dominate
the earth.The first birds evolve from reptiles. Mammals first evolve
135 mya: Cretaceous period begins. Mammals at this
time are small and divided into two major groups- marsupials and
65 mya: Dinosaurs become extinct. Without competition
from the large reptiles, mammals begin to proliferate. Evolution
of flowering plants, or angiosperms.
65 - 1.8 mya
65 mya: Tertiary period
and Paleocene epoch begin. Mammals grow larger and fill the ecological
niches that dinosaurs previously had. The earliest primates evolve.
These primates were small insectivores who were most likely terrestrial.
During this epoch, primates began to include food items such as seeds,
fruits, nuts and leaves in their diet.
53.5 mya: Eocene epoch begins. Primates diversify
and some become arboreal. Primates have developed prehensile hands
and feet with opposable thumbs and toes and their claws have evolved
into nails. Arboreal primates evolve relatively longer lower limbs
for vertical clinging and leaping. Their eye sockets are oriented
more frontally resulting in stereoscopic vision. Primates of this
epoch belong to the prosimian family.
37.5 mya: Oligocene epoch begins. The anthropoid
suborder of primates emerges with the platyrrhines (New World monkeys)
and the Old World monkeys (of the catyrrhine family). The eye sockets
of these primates become completely enclosed with bone and the snout
decreases in size, which implies a greater reliance on sight and
less on smell.
22.5 mya: Miocene epoch begins. Dryopithecines,
the earliest apes (hominid superfamily), evolve in Africa.
5.5 mya: Pliocene epoch begins. Human-like apes
evolve on the African savannas. These apes are bipedal and use tools
to obtain food. Chimpanzee and gorilla ancestors diverge from the
2 mya: Appearance of Homo sapiens (humans).
1.8 mya: Quaternary
period and Pleistocene epoch begin. Humans begin to use stone tools.
10,000 years ago: Holocene epoch begins. Humans
first begin to domesticate plants and animals.