Around 7 million years ago, some of these ape-like creatures moved away from the forests and began living on the grass covered plains. As they spent less time in trees and more time traveling across open ground, evolutionary pressure favored those with longer legs, and they began walking upright.
Their hands were no longer needed for walking and could be used for a variety of other tasks such as gathering food or using sticks and stones as weapons. They could now hunt larger animals and carry the meat back to share with the rest of the group. Being able to provide better care for their increasingly helpless infants allowed them to continue to evolve larger brains that were more adapted towards learning. These animals were the early ancestors of humans.
They began breaking stones apart to create sharp edges which could be used to cut wood, meat, and bone. Around one million years ago, they learned how to keep fires burning and began cooking their meals. Cooked food required less chewing and so their jaws evolved to become smaller.
The competition between different groups of early humans was fierce. Those who could make better weapons, and those who were more skilled at using them had a distinct advantage. Increased competition, better parental care, better communication, and an increasingly complex lifestyle, all of these factors drove the evolution of a larger and more adaptable brain. Groups that evolved larger brains outcompeted and replaced groups that did not.
The shift towards learning was accompanied by an increased sense of curiosity. When one member of a group discovered a new way of doing something, the others would watch and imitate the idea. Older members of the group might be slow to change their habits, but the younger ones would grow up knowing only the new way. Group behavior usually adapts over time to make the most efficient use of the available resources. New discoveries are rare. New ideas usually come through contact with other groups.
Animals communicate by making sounds and using body gestures to express their emotions. As early humans became increasingly skillful, the sounds they made with their mouths became more controlled and meaningful. Grunting noises gradually changed into words. The need for improved voice control led to changes in throat design. These changes made it easier for them to choke while eating or drinking, but improved voice control outweighed this disadvantage.
The immediate advantage of improved communication led to stronger evolutionary pressure to learn more words and develop the creativity to string words together to form sentences. They were now able to learn more from each other and develop closer relationships by sharing personal experiences. And they could now cooperate much more effectively than ever before.
As cooperation between the sexes improved, the female body evolved more towards facilitating reproduction, while the male body evolved for more aggressive and physically demanding tasks such as hunting large animals and protecting the group. Women focused more on maintaining the camp, gathering and preparing food, and caring for family members.
Men and women were drawn together by shared interests and powerful sexual desires. The deeper emotional attachments that grow through familiarity then helped them stay together to share in the training of their children for adult life.
Early humans lived in family tribes that moved around the countryside following herds of wild animals and gathering different fruits as they came into season. They slept in caves or made simple shelters under trees. Clothes were made from animal skins. Hand axes were the all purpose tools. Warm clothes and effective weapons allowed them to slowly migrate to colder climates and less fertile environments.
They spread across Africa, Europe, and Asia in waves of migration that continued for hundreds of thousands of years. Each new wave either wiped out the previous inhabitants or interbred with them. Interbreeding had the advantage that any successful qualities of the new arrivals were combined with native adaptations that suited the local geography and climate. The results can be seen today. Modern humans all share similar abilities to our last common ancestor, but we come in a variety of shapes and colors.
This timeline of evolution of life represents the current scientific theory outlining the major events during the development of life on planet Earth. In biology, evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organization, from kingdoms to species, and individual organisms and molecules, such as DNA and proteins. The similarities between all present day organisms indicate the presence of a common ancestor from which all known species, living and extinct, have diverged through the process of evolution. Although more than 99 percent of all species that ever lived on the planet are estimated to be extinct, there are currently 10–14 million species of life on the Earth
Mantle convection, the process that drives plate tectonics today, is a result of heat flow from the Earth’s interior to the Earth’s surface.:2 It involves the creation of rigid tectonic plates at mid-oceanic ridges. These plates are destroyed by subduction into the mantle at subduction zones. During the early Archean (about 3.0 Ga) the mantle was much hotter than today, probably around 1600 °C,:82 so convection in the mantle was faster. While a process similar to present day plate tectonics did occur, this would have gone faster too. It is likely that during the Hadean and Archean, subduction zones were more common, and therefore tectonic plates were smaller.:258
The initial crust, formed when the Earth’s surface first solidified, totally disappeared from a combination of this fast Hadean plate tectonics and the intense impacts of the Late Heavy Bombardment. However, it is thought that it was basaltic in composition, like today’s oceanic crust, because little crustal differentiation had yet taken place.:258 The first larger pieces of continental crust, which is a product of differentiation of lighter elements during partial melting in the lower crust, appeared at the end of the Hadean, about 4.0 Ga. What is left of these first small continents are called cratons. These pieces of late Hadean and early Archean crust form the cores around which today’s continents grew
One of the reasons for interest in the early atmosphere and ocean is that they form the conditions under which life first arose. There are many models, but little consensus, on how life emerged from non-living chemicals; chemical systems that have been created in the laboratory still fall well short of the minimum complexity for a living organism.
The first step in the emergence of life may have been chemical reactions that produced many of the simpler organic compounds, including nucleobases and amino acids, that are the building blocks of life. An experiment in 1953 by Stanley Miller and Harold Urey showed that such molecules could form in an atmosphere of water, methane, ammonia and hydrogen with the aid of sparks to mimic the effect of lightning. Although the atmospheric composition was probably different from the composition used by Miller and Urey, later experiments with more realistic compositions also managed to synthesize organic molecules. Recent computer simulations have even shown that extraterrestrial organic molecules could have formed in the protoplanetary disk before the formation of the Earth.
The next stage of complexity could have been reached from at least three possible starting points: self-replication, an organism’s ability to produce offspring that are very similar to itself; metabolism, its ability to feed and repair itself; and external cell membranes, which allow food to enter and waste products to leave, but exclude unwanted substances