Types of evidence
These theories can't have come out of thin air...
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Fossils
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Mitochondrial DNA
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Y chromosome
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- Looking at newly discovered fossils and comparing them with existing fossils helps to establish a degree of similarity. For example, over time we know that jaw size became smaller over time due to finding many fossils - the oldest having large jaws and the youngest having smaller jaws. If we find a new fossil, which has a medium-sized jaw, we know that it likely lived between the time the two above fossils lived.
- Fossils that have intermediate structures can be evolutionary 'in-betweens' and show transitions between two species.
- Fossils can easily be dated by dating the type of rock it was found in, which helps give a timeline of evolution.
- Mitochondrial DNA (mtDNA) is incredibly useful for tracing evolution back many generations. It is DNA that is found inside mitochondria, and is passed from mother to child.
- It remains unchanged as it does not undergo meiosis (crossing over, independent assortment, etc.) however is affected by mutations now and then. Mutations in mtDNA occur at a relatively steady rate and this information (the number of mutations/differences between two sets of mtDNA) can be used to estimate how recently two species shared a common ancestor.
- mtDNA only follows the maternal line, so if a female only had male offspring, her mtDNA line would die out. This can limit the knowledge gained from using this technique.
- Scientists use the Y chromosome in the same way as mtDNA - it tracks back through many generations, however in this case it is up the male lineage.
- Like mtDNA, the Y chromosome is not affected by any reshuffling during meiosis (it does not undergo crossing over etc.) and any changes to the DNA sequence are based on mutations that occur at a relatively predictable rate. Scientists look at several polymorphisms found on the Y chromosome to learn about the evolution of humans over time.
There are two main theories of dispersal:
All the genes in modern-day humans (people today) resulted from the mixing of alleles between the European, Asian and African populations of H. sapiens, and that each of these populations evolved in parallel when they diverged 1.8mya.
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All the genes in modern-day humans (people today) came out of Africa as recently as ~65,000ya when modern H. sapiens migrated and replaced all other human populations, wiping out their alleles as there was little interbreeding between old and new populations.
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1. Multiregional theory of dispersal
The multiregional theory of dispersal is based on fossil evidence and comparisons between our modern anatomy compared to our archaic anatomy. It states that:
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This theory of dispersal comes with several predictions:
- Since this theory suggests that there was evolution from H. erectus to H. sapiens in several regions of the world, in all those places we should be able to find transitional fossil forms, as evolution occurs gradually and not in one reproductive cycle (generation), rather over thousands of generations.
- Modern traits common in humans should appear around the same time in all locations of the world, as gene flow was maintained between geographical areas.
- The traits we identify as been 'racially-linked' should be able to be traced back to ancient forms of those particular traits in that particular region.
- Since our most recent ancestor lived around a million years ago, and since then we have become distinct populations with different selection pressures, we have had plenty of time to accumulate numerous genetic differences, suggesting we should be quite genetically varied as a human group.
- The amount of genetic variation within our regional areas should be approximately the same, since we have all been evolving together.
2. Replacement theory of dispersal
The replacement theory of dispersal is also called the Out of Africa Hypothesis, as well as the Eve Hypothesis as it is based on mitochondrial DNA evidence leading back to a 'mitochondrial Eve' (ancestral mother). This theory states that:
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This theory of dispersal comes with several predictions:
- Since this theory suggests that modern humans came from one H. sapiens population from Africa, all transitional forms between H. erectus and H. sapiens should be found in Africa.
- Modern characteristics should first appear in Africa, where they are thought to have originated, and as H. sapiens spread out all over the world (slowly), these characteristics should be found elsewhere a bit later on (not all at the same time).
- Modern populations should overlap with existing/archaic populations in places such as Europe and Asia, as populations would occupy a similar geographical area until the modern H. sapiens actually replaced the existing populations (it didn't just happen straight away, it took time for H. sapiens to out-compete existing populations).
- Humans today should be quite genetically similar, as our most common ancestor was alive around 100,000. 100,000 years is not long enough to accumulate substantial genetic differences.
- The regional populations we have today should have different amounts/levels of genetic diversity, relative to the time modern H. sapiens have been living in those locations. For example, H. sapiens were first in Africa, so the genetic diversity in Africa should be larger than that of China, which was colonized much later in comparison, so has had less time to accumulate many genetic differences.
Keep this quote in mind (from The Book of Man)
"Living genes must have ancestors, whereas dead fossils may not have descendents. Molecular biologists therefore know the genes they are examining must have been passed through lineages that survived to the present; paleontologists cannot be sure that the fossils they examine do not lead down an evolutionary 'blind alley'."
What the evidence found so far suggests:
Analysis of fossils in humans shows that:
Analysis of mtDNA / Y chromosome in humans shows that:
- Quite a large number of transitional fossils have been found outside of Africa, many in China. The idea of H. erectus transitioning into H. sapiens out of Africa supports the Multiregional theory. (Replacement theory says H. erectus evolved into H. sapiens in Africa, then left).
- Fossil evidence shows that modern H. sapiens occupied areas of the Middle East before H. neanderthals arrived, there - which knocks down the idea that H. sapiens evolved from H. neanderthals - an idea stemming from the Multiregional theory. This therefore supports the Replacement theory, suggesting modern H. sapiens replaced any existing hominin groups, such as the Neanderthals (which became extinct anyway).
Analysis of mtDNA / Y chromosome in humans shows that:
- the highest level of genetic variation within a regional population is found in African populations. This suggests that these populations have existed longer than any other population, because the more time a population is alive for, the more genetic differences it can accumulate. A large amount of genetic variation shows a large number of differences, therefore you can infer that this population has been around for longer than any other. This supports the Replacement theory.
- mtDNA accumulates mutations/differences at a relatively steady rate, and using this information scientists can create a 'molecular clock' to gauge how long H. sapiens have been around for - which they think is between 400,000 - 100,000 years. This supports the Replacement theory.
- There's a low level of genetic diversity on modern-day humans, suggesting we came from one relatively small population (10,000 to 50,000) of hominins who left Africa - a founder population who may have experienced a bottleneck effect on their journey to colonise the world! However, the differences we see are based on variations of characteristics based on our geographic location - e.g. darker skin typical of those living in tropical areas). This supports the Replacement theory.
- Argument that mtDNA is flawed because if it is not present today, it can't ever be traced back to a time when perhaps there was a link (i.e. present-day patterns cannot show any links that became extinct). This argument is in support of the Multiregional theory.
- All modern humans can trace their evolutionary heritage to a single woman via changes in mitochondrial DNA. This woman, 'Eve', lived about 200,000 years ago in Africa. This supports the Replacement theory.
Most evidence found (fossils, mtDNA, Y chromosme DNA) supports the Replacement theory. This is therefore the more widely accepted of the two theories.