By Maina Kiarie
Everything we see today; vegetation, animals, insects, man, has an origin. Evolution means the process by which different kinds of living organisms are thought to have developed and diversified from earlier forms during the history of the earth. Evolution refers to any change across successive generations in the heritable characteristics of biological populations.
As human beings we are drawn by curiosity to unravel the mysteries of our surroundings and get the answers that explain why things are the way they are. From the origin of the universe, life on earth, man, animals, plants and so on. Like everything else around us, human beings too have an origin and much study has gone into gathering knowledge and evidence to support each theory of human origin that has been put across over the years.
Charles Darwin was the first to formulate a compelling scientific argument for the theory of evolution by means of natural selection. Evolution by natural selection is a process that is inferred from three facts about populations:
- more offspring are produced than can possibly survive,
- traits vary among individuals, leading to differential rates of survival and reproduction, and
- trait differences are heritable.
It follows that when members of a population die they are replaced by the progeny of parents that were better adapted to survive and reproduce in the environment in which natural selection took place. This process preserves traits that are seemingly fitted for the functional roles they perform. Natural selection is the only known cause of adaptation, but not the only known cause of evolution. Other, nonadaptive causes of evolution include mutation and genetic drift.
The human evolution theory seeks to explain the origin of Homo sapiens sapiens (modern man) as a distinct species from other hominids, great apes and placental mammals. The origins of human evolution date back to more than 7 million years ago when the first ape-like species called Sahelanthropus tchadensis lived, paving way for the evolution of modern man.
However, life on earth began millions of years before the existence of the Sahelanthropus tchadensis, the ancestor of modern man. The earth is believed to have been formed 4570 billion years ago (Ga) followed by the moon around 4533 Ga in the Cryptic era, Hadean eon. The first lifeforms and self-replicating RNA molecules evolved around 4000 Ma. followed by the evolution of simple single-celled life, probably bacteria and perhaps archaea in the Eoarchean era 3800 Ma. A cell is the basic unit of life. By the time a human being reaches adulthood, the body consists close to 50 trillion cells, which are organised biologically to eventually form the whole body.
In 2050 Ma in the Orosirian period the atmosphere became oxygenic. The first complex single-celled life (protists with nuclei) evolved 70 million years later in the Paleo-proterozoic era in the Statherian period (1800 Ma). Fossils of the first multi-celled animals are dated about 630 Ma in the Ediacaran period in the Neo-proterozoic era. In the early epoch of the Cambrian period the atmospheric carbon dioxide content was roughly 20-35 times the present day.
In the Devonian period (416.0-374.5Ma) characterised the highest-ever atmospheric oxygen levels. The Jurassic period saw the existence of mammals albeit small in size, the first birds and lizards. The atmospheric carbon dioxide (CO₂) levels were 4-5 times the present day levels. In the Paleocene epoch, following the extinction of dinosaurs, mammals diversified into a number of primitive lineages and the first large mammals -up to bear or small hippo size evolved.
Paleocene epoch was followed by the Eocene epoch (55.8-37.2 Ma) during which time several “modern” mammal families appeared as did the first grasses. Modern mammal and bird families became recognisable in the Miocene epoch (23.03-7.246Ma). The first apes-modern man’s ancestor, the Sahelanthropus tchadensis evolved during this epoch around 7 Ma. Widespread forests slowly drew in massive amounts of carbon dioxide, gradually lowering the level of atmospheric CO₂.
Many of the existing genera of mammals evolved in the Pliocene epoch (5.332-3.600 Ma). Also, the Australopithecines, another of man’s ancestors evolved at this time as did the first Homo species, the Homo habilis. This period is soon approaching the modern geologic age, Holocene.
Evolution of anatomically modern humans occurred in the Pleistocene epoch (2.588-0.126 Ma), the time period between Pliocene and Holocene epoch. Human stone age cultures evolved at this time with increasing technical complexity.
Human civilization from the stone age cultures rose around 11,430 years ago and continues in the present day Holocene epoch. Palaeolithic/Neolithic stone age cultures 12,010 years ago, gave way to Copper age 5510 years ago and Bronze age 4510 years ago. Human cultures continue to grow in complexity and technical advancement through the iron age (3210 years ago), giving rise to many pre-historic cultures throughout the world. Atmospheric CO₂ gradually rose from the all time low 100 ppmv (parts per million volume) in the Miocene epoch to the current level of 385ppmv.
Fossils found in various places across the world tell the story of our species, a story spanning millions of years. Every fossil provides valuable insights into the world of our ancestors. According to the genetic and fossil evidence, archaic Homo sapiens evolved anatomically modern humans solely in Africa between 200,000 and 100,000 years ago.
How did the early man spread across the world? Most scientists ascribe to the Out of Africa migration hypotheses to describe how the early human populations spread from their ancestral land, Africa. This theory states that the historical migration of early human populations began in Africa when the Homo erectus first migrated out of Africa over the Levantine corridor and Horn of Africa to Eurasia about 1.8 million years ago.
Apes look remarkably like humans, with the DNA of our closest cousin, the chimpanzees approximately 98.4% identical to man’s. Owing to our similar ancestry, human beings share many adaptations with other living primates. However, despite belonging to the same family, Hominidae, human beings have lost some adaptations on their evolutionary journey such as the ability to jump from one tree branch to another and very hairy bodies. The living great apes are still adapted to an arboreal lifestyle (life in the trees).
Humans and other hominids evolved a unique skeletal adaptation that enabled them walk upright. This ability to walk upright was later followed by other human evolutionary trends that led to tool manufacture and enlargement of the brain.
With the changes in the environment over the evolutionary process some species adapted and survived while those who failed to adapt are now extinct. Of the Homo species only Homo sapiens sapiens (modern man) exists today. It leaves one to wonder whether modern man is still evolving and will turn into something else!