The Toba Catastrophe Theory
74,000 years ago one of the biggest volcanic eruptions of the past 2 million years occurred in Indonesia. The super-eruption caused a large proportion of the Indonesian island, Sumatra to explode and caused 2,800 cubic kilometres to be covered in volcanic material, killing all living organisms surrounding the volcano. The force from the eruption caused tsunamis. Billions of tonnes of ash and rock were ejected into the atmosphere, as well as a deafening noise. The gases and ash that were released during the eruption were spread around the globe via wind. The gases and ash reflected the sun’s heat and prevented it from entering the Earth's atmosphere, thus affecting rainfall and climate all around the world. Some scientists believe that this change in temperature and rainfall caused a genetic bottleneck to occur in all the Homo species living during that era, which included Homo neanderthalensis, Homo sapiens and Homo floresiensis (Haslam and Petraglia, 2009). However, many scientists have disproven this theory and identified that some species endemic to areas around Sumatra and Indonesia may have experienced a genetic bottleneck, which resulted in their extinction. Thus, the Toba super-eruption was not the event that almost wiped out all of the Homo species 74,000 years ago.
Gene Flow (Migration)
Gene flow is the movement of alleles into (immigration) or out of (emigration) a population. When gene flow occurs, and a small, unrepresentative group of individuals emigrate to colonise a new region, their new population will experience a high level of genetic drift. Thus, the allele frequency of the new population is substantially different to the allele frequency of the original populations gene pool. This type of genetic drift is referred to as the founder effect and results in little genetic diversity within the founding population. A lack of genetic diversity makes individuals of a population more susceptible to selection pressures, such as disease, which increases the species chance of becoming extinct. Therefore, when the different species of homo emigrated out of Africa, the founding populations may have experienced high levels of genetic drift, which lowered the genetic diversity of their population. Thus, this would have made the different species of Homo to become more susceptible to selection pressures, which could have contributed to the extinction of some of the Homo species.
The out of Africa theory is illustrated on the right-hand side of figure 4.02 below and suggests that Homo sapiens evolved in Africa 200,000 years ago and then emigrated out of Africa 100,000 to 50,000 years ago, replacing existing hominin species in Europe and Asia. Whereas the multi-regional theory depicted on the left-hand side of figure 4.02 indicates that Homo erectus emigrated out of Africa and immigrated into Eurasia 1 million years ago. The multi-regional theory suggests that the evolution of Homo erectus into Homo sapien was a continuous process that occurred across all regions since gene flow was occurring between different continental populations.
The out of Africa theory is illustrated on the right-hand side of figure 4.02 below and suggests that Homo sapiens evolved in Africa 200,000 years ago and then emigrated out of Africa 100,000 to 50,000 years ago, replacing existing hominin species in Europe and Asia. Whereas the multi-regional theory depicted on the left-hand side of figure 4.02 indicates that Homo erectus emigrated out of Africa and immigrated into Eurasia 1 million years ago. The multi-regional theory suggests that the evolution of Homo erectus into Homo sapien was a continuous process that occurred across all regions since gene flow was occurring between different continental populations.
Interbreeding
|
Interbreeding is the process by which members of the same species reproduce via sexual reproduction to produce viable, fertile offspring. However, interbreeding can also occur between members of different sub-species or species. For example, dogs and wolves can be mated together to produce fertile offspring since they are the same species. However, zebras and donkeys can be breed together to produce offspring, but the offspring are infertile (unable to reproduce) since they are members of different species.
Genetic scientists conducted an experiment in 2010 in which they sequenced and compared the genomes of 5 living individuals from China, France, Papua New Guinea, southern Africa and western Africa to the genome of a neanderthal. The study identified that the DNA of neanderthals is 99.7% identical to the DNA of modern Homo sapiens. Furthermore, individuals from different ethnic backgrounds, excluding Africans, carry traces of neanderthal DNA in their genome. Since there is no fossil evidence to support that neanderthals live in China or Papua New Guinea, scientists concluded that members of Homo neanderthalensis must have entered populations of Homo sapiens just after Homo sapiens emigrated out of Africa, but before they immigrated around the globe. The results led the scientists to believe that members of Homo neanderthalensis must have interbreed with members of Homo sapiens 60,000 years ago since there is fossil evidence to support that these two species were living in the same region during this time period (Than, 2010). Therefore, this interbreeding between neanderthals and modern humans may have increased the genetic diversity of Homo sapien populations, but may have decreased the genetic diversity of Homo neanderthalensis populations, which may have promoted the extinction of neanderthals. |
Inbreeding
Inbreeding involves two closely related individuals of the same species mating to produce offspring. Inbreeding decreases the number of heterozygous individuals and increases the number of homozygous individuals in the populations gene pool. This results in a higher chance of offspring expressing homozygous recessive conditions in their phenotype, whilst decreasing the genetic diversity of the population.
Inbreeding may have occurred between closely related members of Homo sapiens or other Homo species, which would have increased the rate of homozygous recessive phenotypes appearing in the population, whilst decreasing the genetic diversity of the populations gene pool. Therefore, the homozygous recessive conditions may have been selected against by natural selection and individuals with the condition would be less likely to survive, reproduce and pass their alleles to the next generation. In addition, if there was little genetic diversity within the populations gene pool, members of the population would be more susceptible to selection pressures and the chance of the species going extinct would increase.
Therefore, as hominins diverged to form the genus Homo, inbreeding could have occurred between members within each of these Homo species. This inbreeding would lower the genetic diversity within each species and make them more susceptible to selection pressures. For example, closely related members of the species Homo erectus could have inbreed. Some members within the Homo erectus population may have contracted a contagious disease, which could kill all the members of that population due to the lack of genetic diversity.
Inbreeding may have occurred between closely related members of Homo sapiens or other Homo species, which would have increased the rate of homozygous recessive phenotypes appearing in the population, whilst decreasing the genetic diversity of the populations gene pool. Therefore, the homozygous recessive conditions may have been selected against by natural selection and individuals with the condition would be less likely to survive, reproduce and pass their alleles to the next generation. In addition, if there was little genetic diversity within the populations gene pool, members of the population would be more susceptible to selection pressures and the chance of the species going extinct would increase.
Therefore, as hominins diverged to form the genus Homo, inbreeding could have occurred between members within each of these Homo species. This inbreeding would lower the genetic diversity within each species and make them more susceptible to selection pressures. For example, closely related members of the species Homo erectus could have inbreed. Some members within the Homo erectus population may have contracted a contagious disease, which could kill all the members of that population due to the lack of genetic diversity.
Adaptations
Adaptations are adjustments or changes to the behaviour, physiology or structure of an organism to become more suited to a particular environment. The mind map below illustrates some of the most crucial adaptations that hominins have undergone over time to become more suited to the environment in which they were living in. An inability to become more suited to the environment could have contributed to the extinction of some hominin species.