MODELS OF EVOLUTION
As delving into the evolution of the modern humans requires the analysis of data that dates back 8 million years ago, there are varying beliefs and interpretations of the data that has been collected over the years. Such variance in understand leads to differing models of evolution. Presently, there is one theory that is supported however, there is a second that had been vastly considered and accepted in the past.
Multi-Regional Hypothesis
- Such a hypothesis theorises that Homo erectus left Africa around 2 million years ago and resultantly inhabited all regions of the Old World. Since the migration, the process of human evolution has been a continuous one across all of the World, with gene flow present between regional populations.The one differing belief compared to the Out-of-Africa Model is that the evolution of Homo sapiens was not limited to a specific area, instead, was a process that occurred throughout the entire geographical range in which was inhabited by humans (see figure 1.1).
Figure 1.1: Multiregional origin of modern humans. (2015, September 9).
Retrieved September 21, 2015, from https://en.wikipedia.org/wiki/
Multiregional_origin_of_modern_humans
Retrieved September 21, 2015, from https://en.wikipedia.org/wiki/
Multiregional_origin_of_modern_humans
Out-of-Africa Model
- Theorises that Homo erectus also migrated to the Eurasia region around 1 million years ago, however, proposes that Homo sapiens evolved in Africa 200,000 years ago. Homo sapiens then began to migrate out of Africa 70,000-50,000 years ago and by doing this, eventually replaced all other hominid species in the Europe and Asia regions (see figure 1.2).
Figure 1.2: A.Furuti, C. (2012, August 28). Map Projections: Some
Applications. Retrieved September 25, 2015, from http://www.progonos.
com/furuti/MapProj/Dither/ProjAppl/projAppl.html
Applications. Retrieved September 25, 2015, from http://www.progonos.
com/furuti/MapProj/Dither/ProjAppl/projAppl.html
What to believe?
The main forms of evidence that is used presently to analyse which model of evolution is correct is genetic evidence and anatomical evidence. (Donald Johanson, Director of the Institute of Human Origins at Arizona State University)
Genetic Evidence
- Smaller facial skeleton with a flatter face
- A less robust skeleton
- Larger stature
Genetic Evidence
- Through the investigation of the patterns of genetic variation with human populations, the Out of Africa view, that the evolution of Homo sapiens is a recent event, is supported.
- Studies of mitochondrial DNA reveal that humans have very little genetic variation, implying that the evolution of Homo sapiens was a recent event
- Africans display a higher genetic variation compared to other populations, supporting the hypothesis that the Africans were the first modern humans.
- The highest mitochondrial DNA variance occurs in African populations. This allows for the assumption that Homo sapiens arose in Africa and, the fact that the variance is larger, implies that those of African populations have had a longer time to cumulate genetic diversity. Additionally, through the genetic distance between African populations and modern populations, it can be measured as an estimate of time that Homo sapiens arose between 100,000 and 400,000 years ago in Africa.
- In recent times, Neanderthal DNA has been extracted from skeletons, and after investigations into the mitochondrial DNA, it has been determined that Neanderthal DNA is distinctively different to the DNA of Homo sapiens. Through the analysis of the genetic difference in reference to time, it is suggested that the two lineages split more than 400,000 years ago.
- The Out of Africa Model suggests that over 1 million years ago, Homo erectus migrated from Africa, moved around the world, through processes of genetic drift and natural selection, the morphology of geographical populations became more diverse. In Asia, Homo erectus was prominent, whereas in Europe, Neanderthals were inhabitants.
- As Neanderthals within Europe were subject to a colder climate, they are now distinguishable by certain anatomical characteristics:
- Double arched brow ridge
- Larger facial skeleton with projecting mid-face and large nasal cavity
- Larger dentition with larger molars and incisors
- Short forearms and lower legs
- Short stature
- Large brain
- Oddly shaped occipital region at back of skull that formed a bulge’ - As the European Neanderthals were geographically isolated for such a long periods of time (around 130,000 years ago), they were vastly anatomically different, resulting them now being classified as a separate species- Homo neanderthalensis.
- At around this time, in Africa, the early Homo sapiens appeared with a differing anatomical structure.
- Smaller facial skeleton with a flatter face
- A less robust skeleton
- Larger stature
- Such anatomical evidence leads to the suggestion that Homo sapiens and Homo neanderthalensis had been isolated from each other and evolved without interbreeding, leading to the conclusion that Homo sapiens evolved in Africa, whilst isolated from the Homo neanderthalensis species.
- Presently, the Out of Africa is widely accepted as the anatomical and genetic evidence suggests that the evolution of modern humans is a rather recent event.