Theories
Multiregional hypothesis
This theory is based on fossil evidence and the anatomical characteristics of the modern population.
The theory states that humans (H. erectus) first left Africa approximately 2 million years ago and went to Europe and Asia. Homo sapiens emerge gradually throughout the world, and a s populations dispersed, they remained in genetic contact with each other. Gene flow between neighbouring populations ensured that the general "modern human blueprint" was adapted by all. However, as gene flow was limited, this led to slight anatomical differences which were developed/retained in regional populations and seen in fossil evidence.
This theory is based on fossil evidence and the anatomical characteristics of the modern population.
The theory states that humans (H. erectus) first left Africa approximately 2 million years ago and went to Europe and Asia. Homo sapiens emerge gradually throughout the world, and a s populations dispersed, they remained in genetic contact with each other. Gene flow between neighbouring populations ensured that the general "modern human blueprint" was adapted by all. However, as gene flow was limited, this led to slight anatomical differences which were developed/retained in regional populations and seen in fossil evidence.
Out of Africa hypothesis (Replacement hypothesis)
This theory is based largely on genetic studies of modern human DNA. This model states that modern humans (H. sapiens) are a separate species and that modern humans evolved from earlier species in only one location- Africa 170,000 years ago. Modern humans left Africa around 40,000 years ago, replacing the earlier species they came in contact with (e.g erectus and heidelbergensis). The theory states that there was no gene flow between the African, European and Asian populations.
Evidence for this theory involves:
This theory is based largely on genetic studies of modern human DNA. This model states that modern humans (H. sapiens) are a separate species and that modern humans evolved from earlier species in only one location- Africa 170,000 years ago. Modern humans left Africa around 40,000 years ago, replacing the earlier species they came in contact with (e.g erectus and heidelbergensis). The theory states that there was no gene flow between the African, European and Asian populations.
Evidence for this theory involves:
- Mitochondrial DNA (mtDNA): (mtDNA is not expressed in phenotypes so is not affected by natural selection) mtDNA is inherited only from the mother. mtDNA can therefore only be changed by mutation (unlike nuclear DNA which is altered through processes such as independent assortment and crossing over therefore is not as useful). Mutation occurs at a known rate in mtDNA, 10x faster than nuclear DNA (as it does not have the elaborate "proof reading" mechanisms of nuclear DNA). A given amount of change occurs in one tenth of the time of nuclear DNA do it acts as a molecular clock. If we know how much mtDNA from a range of individuals differs, we can work out how long it would have taken for that variation to arise. This is how the date of 170,000 years ago for the "mitochondrial Eve" was determined. Therefore mtDNA allows us to trace back to when populations last had a common ancestor. This theory states that all modern descendants contain mtDNA that can be traced back to Eve.
- y chromosome evidence: Y- chromosomes exhibit some of the same conservative properties that mtDNA does, as they have no homologous partner (a male has one Y chromosome only: XY, and females have none: XX). Variation in the DNA contained in the Y chromosome can therefore also be used to trace evolutionary relationships. Analysis of known variations in the Y chromosome DNA called "markers" that are associated with known mutations can inform us about the origins of modern humans.
- A mutation occurred in an African Y chromosome about 44,000 years ago. If modern humans had originated from African populations since that mutation then one would expect modern Y chromosomes to possess that same mutation. Scientists have tested this premise by sampling DNA from over 12,000 individuals from 163 current populations in Asia. Each and every single individual showed the particular Y chromosome mutation in question and there were no mutations possessed by modern Asians that Africans did not also have, which suggests they all descended from an African origin after the advent of that mutation 44,000 years ago.
- Nuclear DNA evidence: When humans left Africa they left as a founder population. Founder populations have a smaller gene pool/show less genetic variation. When compared- African populations show more variation than European or Asian populations.
Hybridisation model
This is a varied form of the "Out of Africa" hypothesis. The Hybridisation model states that modern humans evolved in Africa only 170,000 years ago and migrated out of Africa approximately 40,000 years ago when they left Europe and Asia.
Denisovans shared and ancestor with modern humans and Neanderthals about 9000,000 to 1,000,000 years ago. Modern humans and Neanderthals had a common ancestor approximately 500,000 years ago. Neanderthals and Denisovans last shared an ancestor 300,000 years ago. Modern humans last shared a common ancestor 200,000 years ago (mitochondrial Eve).
Modern humans interbred with other human populations already in Europe and Asia (ie they interbred with Neanderthals and Denisovans).
Evidence
Neanderthal hybridisation: All non-African populations today contain 1-4% Neanderthal DNA. (Meaning in terms of dispersal) Because only European and Asian populations today have Neanderthal DNA, interbreeding with Neanderthals must have occurred in the Middle East about 80,000 years ago.
Denisovans hybridisation: Modern day Melanesians contain 4-6% of Denisovan DNA. Because of this interbreeding between H. sapiens and Denisovan probably occurred in Siberia producing hybrid offspring en route to migrating to Melanesia.
This is a varied form of the "Out of Africa" hypothesis. The Hybridisation model states that modern humans evolved in Africa only 170,000 years ago and migrated out of Africa approximately 40,000 years ago when they left Europe and Asia.
Denisovans shared and ancestor with modern humans and Neanderthals about 9000,000 to 1,000,000 years ago. Modern humans and Neanderthals had a common ancestor approximately 500,000 years ago. Neanderthals and Denisovans last shared an ancestor 300,000 years ago. Modern humans last shared a common ancestor 200,000 years ago (mitochondrial Eve).
Modern humans interbred with other human populations already in Europe and Asia (ie they interbred with Neanderthals and Denisovans).
Evidence
Neanderthal hybridisation: All non-African populations today contain 1-4% Neanderthal DNA. (Meaning in terms of dispersal) Because only European and Asian populations today have Neanderthal DNA, interbreeding with Neanderthals must have occurred in the Middle East about 80,000 years ago.
Denisovans hybridisation: Modern day Melanesians contain 4-6% of Denisovan DNA. Because of this interbreeding between H. sapiens and Denisovan probably occurred in Siberia producing hybrid offspring en route to migrating to Melanesia.
Reasons and Implications
Why disperse
In our evolutionary past there must have existed significant reasons to leave Africa and disperse. There is a cost involved with such movement, so to make it worthwhile there must also have been a benefit (or, more precisely, a need).
Earth has undergone many cycles of warming and cooling and these changes in climate affect the location and availability of plant life.
A cooler, dryer savannah has less available food than a warmer, wetter rainforest.
Food shortages would almost definitely have forced our ancestors to move to other areas where food was more readily available to support the population. Saharan Africa changed from warm and wet to cool and dry and arid several times in the recent past:
At these times Hominids would have been forced into other wetter areas of Africa or into Europe and Asia to survive.
In our evolutionary past there must have existed significant reasons to leave Africa and disperse. There is a cost involved with such movement, so to make it worthwhile there must also have been a benefit (or, more precisely, a need).
Earth has undergone many cycles of warming and cooling and these changes in climate affect the location and availability of plant life.
A cooler, dryer savannah has less available food than a warmer, wetter rainforest.
Food shortages would almost definitely have forced our ancestors to move to other areas where food was more readily available to support the population. Saharan Africa changed from warm and wet to cool and dry and arid several times in the recent past:
- 1.7 million years ago
- 500,000 years ago
- 60,000 years ago
At these times Hominids would have been forced into other wetter areas of Africa or into Europe and Asia to survive.
Dispersal routes
Some evidence suggests that major dispersal routes may have initially been along the coast and around the equator. This explains how Australia was populated around the same time as Europe, despite being substantially further away from Africa.
Some evidence suggests that major dispersal routes may have initially been along the coast and around the equator. This explains how Australia was populated around the same time as Europe, despite being substantially further away from Africa.
Dating Fossils
Relative dating
This method puts fossils in order of age without knowing the actual age.
This method puts fossils in order of age without knowing the actual age.
Absolute dating
Dating a fossil in years e.g using 14C dating (see below)
Carbon 14- Method: Measures the amount of radioactive 14C remaining in the fossil sample. Material: organic matter. Age limit: 500-50,000 years (the background and natural radiation interfere)
Potassium-argon- Method: used to date volcanic material around the fossil. Material: Volcanic rock. Age limit: Over 400,000 years. Comment: only useful for fossils in undisturbed sites in volcanic rock.
Fission track- Method: When Uranium-238 decays to lead, the U atoms emit high energy particle which make tracks about 10 micrometres long in the surrounding rock. These can be seen under the microscope after the rock is exposed to chemicals. The number of tracks enables the proportion of Uranium atoms that have decayed to be calculated and hence the age of the rock. Material: Volcanic rock. Age limit: Over 10,000. Comments: Useful as independent of K-Ar dating. Can be used only for volcanic material.
Thermo luminescence- Method: Clays contain traces of radioactive elements that emit electrons as they decay. The longer the time since the pottery was last heated the more electrons are trapped and more light will be emitted when heated in a laboratory. Material: Pottery and charred flint. Age limit: no limits. Comments: Pottery must be fired- cannot be replaced.
Dating a fossil in years e.g using 14C dating (see below)
Carbon 14- Method: Measures the amount of radioactive 14C remaining in the fossil sample. Material: organic matter. Age limit: 500-50,000 years (the background and natural radiation interfere)
Potassium-argon- Method: used to date volcanic material around the fossil. Material: Volcanic rock. Age limit: Over 400,000 years. Comment: only useful for fossils in undisturbed sites in volcanic rock.
Fission track- Method: When Uranium-238 decays to lead, the U atoms emit high energy particle which make tracks about 10 micrometres long in the surrounding rock. These can be seen under the microscope after the rock is exposed to chemicals. The number of tracks enables the proportion of Uranium atoms that have decayed to be calculated and hence the age of the rock. Material: Volcanic rock. Age limit: Over 10,000. Comments: Useful as independent of K-Ar dating. Can be used only for volcanic material.
Thermo luminescence- Method: Clays contain traces of radioactive elements that emit electrons as they decay. The longer the time since the pottery was last heated the more electrons are trapped and more light will be emitted when heated in a laboratory. Material: Pottery and charred flint. Age limit: no limits. Comments: Pottery must be fired- cannot be replaced.