Charles Darwin (1809 - 1882) shook the scientific world with his publication of the Origin of Species by Natural Selectionin 1859. Pointing to variability within species, Darwin observed that while offspring inherit a resemblance to their parents, they are not identical to them. He further noted that some of the differences between offspring and parents were not due solely to the environment but were themselves often inheritable. Animal breeders, he observed, were often able to change the characteristics of domestic animals by selecting for reproduction those individuals with the most desirable qualities, such as speed in racehorses, milk production in cows, trail scenting in dogs. This is change by artificial selection.
	Darwin reasoned that, in nature, individuals with qualities that made them better adjusted to their environments or gave them higher reproductive capacities would tend to leave more offspring; such individuals were said to have higher fitness. Because more individuals are born than survive to breed, constant winnowing of the less fit-a natural selection-should occur. Today we know Darwin's theory as "survival of the fittest."
	Many of Darwin's ideas were formulated during the voyage of the HMS Beagle. Nominated by Botanist John S. Henslow, Darwin served as the ship's naturalist during a 5 year expedition that took him around the world. The voyage gave him ample opportunity to explore, survey and collect data. During his trip, Darwin spent five weeks at the Galapagos Island, which are nearly 500 miles west off the coast of Equador. Darwin made several observations about the numerous different species of tortoises and sparrow-like finches that inhabited the islands. He was convinced that the each species of animal was derived from a common ancestor. To explain the diversity of the species, he reasoned that the animals had changed over time - EVOLVED..

The key that unlocked Darwin's thinking about how evolution takes place was an essay written in 1798 by English economist Thomas Malthus. Malthus wrote that human populations are able to increase faster than the food supply. He suggested that the human population would cover Earth's entire surface within a short period of time if it could reproduce unchecked. Darwin realized that Malthus's ideas about human population apply to all species. Darwin made a key association: Individuals that have physical or behavioral traits that better suit their environment are more likely to survive and reproduce than those that do not have such traits.

Hominid Evolution

In 1871, 12 years after The Origin of Species, Charles Darwin published The Descent of Man. He proposed that humans, gorillas, and chimpanzees all evolved from a common ancestor. The early primates probably walked on all four limbs, but what caused our ancestors to stand up and become two-legged? Fifteen million years ago, the world's climate began to cool, and the great forests of Africa were largely replaced by treeless plains (savannas). To move from one tree to another tree, early primates were forced to walk. Our earliest known direct ancestors belong to the group Australopithecus. Fossils indicate that australopithecines had two key homonid characteristics, 1. bipedal (walking upright) and 2. and a large brain. As the human species slowly evolved, cranial capacity increased allowing for greater reasoning capabilities. Below are diagrams showing how the skull increased in size from early australopithecines to modern homo sapiens.

	Australopithecus anamensis- This species existed from 4.2 - 3.9 million years ago. It was found by Meave Leakey in East Africa and was named in August of 1995. Australopithecus afarensis- existed from 4-3 million years ago. It was found in Ethiopia, Tanzania, and Kenya. Characteristics • cranial capacity of 430 - 440 ml • megadont (huge) teeth • ape-like, long, curve phalanges The most famous fossil evidence is 1) The Laeotoli Footprints found by Mary Leakey. These footprints were evidence of bipedalism. They had defining characteristics such as a strong heel strike, splayed toes, and a slight arch. 2) "Lucy" is the most well known fossil in the world today. Donald Johanson found her in Afar, Ethiopia in 1978. Lucy is the center of many debates. When this fossil was found, it was amongst a group of other A. afarensis. After studying all of the individuals, Johanson believes that this species is sexually dimorphic. Sexual dimorphism is the difference in size between males and females. In this case, the males seem to be more robust, are prognathic, and have bigger canines.
	Australopithecus africanus - This species existed from 3.0-2.3 million years ago. It was found in South Africa. Characteristics: • cranial capacity of 440-480 cm3 • smaller canines than A. afarensi • ape-like arms and legs (arms longer than legs)
	Homo habilis - these species existed 2.3 - 1.6 million years ago. They were found in Tanzania, Kenya, Ethiopia, South Africa, and China. These hominids are considered the first tool users- "habilis means handy human". They used tools that were chips of rock called Oldowan tools. Characteristics: • cranial capacity of 630 cm3 • smaller back teeth • flatter face • no brow ridge • arm still long
	Homo erectus - existed 1.8 million years ago. The separation Characteristics: • cranial capacity of 800-1100cm3 • sloping forehead • big brow ridge • large orbits, nasal openings, and face • thick cranium
	Homo neanderthalensis - This species existed 130,000 to 25,000 years ago. Some scientists propose to push back the date to as far as 225,000 years ago. This species has been found in Europe, East Asia, and the Middle East. Characteristics: • cranial capacity of 1,300-1,750cm3 • double arch brow ridge • sloping forehead • high, wide, nasal openings • no chin • big long incisors • robust skeletons • big arm and leg bones • short stature (avg. 5'5") H. neanderthalensis was found with a more advanced set of tools. These tools are considered more advanced because more work was put into detailing them with animal bones and horns. Neanderthals had: • sophisticated tools • burial sites with tools, animal bones & horns & flowers • cared for the disabled • clothes • shelters • culture: clan of the cave bear • art and music