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The Origin Of Modern Humans. Background Information. First species of Homo, Homo habilis, evolved in Africa around 2 million years ago. Later, a descendant of Homo habilis, Homo erectus evolved (along with other hominids), and spread out of Africa.
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Background Information • First species of Homo, Homo habilis, evolved in Africa around 2 million years ago. • Later, a descendant of Homo habilis, Homo erectus evolved (along with other hominids), and spread out of Africa. • Homo erectus (and its lineage) gave rise to Homo sapiens around 100,000 to 200,000 years ago.
Two Main Theories • Out of Africa Theory (OOA) – suggests that H erectus migrated from Africa about 1mya and formed additional populations in Europe and Asia. Only the African population evolved into H sapiens and again migrated into Europe and Asia replacing H erectus as they went. • Multi-regional Evolution Theory –suggests that H erectus migrated from Africa about 1mya and formed additional populations in Europe and Asia. Parallel evolution occurred and they all became H sapiens with regional differences.
Why migrate? • Scientists think the reason for migration was because of numerous ice ages. These caused Africa to dry and minimised food supply. • The options were to stay and survive, or migrate and look for more opportunity. Migrating also reduces competition. • These ice ages assisted migration as sea levels were lower so land bridges were created. Straights between islands would have been smaller (small boats could be made to cross these?). • Following the coast meant food supply was high. • Migration would have been fast • NCEA 2007 Explain the impact a change in climate from wet tropical to cooler drier conditions would have on migration out of Africa by H. erectus. Discuss the impact a change in climate from wet tropical to cooler drier conditions would have on migration out of Africa by H. erectus.
Modern humans reached Australia as long as 60 000 years ago. This was at much the same time as they were moving into Europe, despite the fact that Europe is much closer to Africa. Explain why modern humans were relatively slow to colonise Europe. • The route into Asia may have been easier to travel and to access food • Europe, being in an ice age would have been heavily glaciated • Neadertals were already present in Europe, increasing competition • Humans adaptations were not suited to cold climates.
Genetic Tools to Find the Answer • Fossil records • DNA sequencing • Mitochondrial DNA analysis (mtDNA) • Maternally inherited, therefore telling the story from the female side of human history • Y Chromosome analysis • Inherited down the paternal line, complementing the mtDNA • Microsatellite DNA analysis • Segments of tandemly repeated DNA with a short repeat length, usually 2-5 nucleotides
NCEA 2011 • Investigations into the patterns of genetic variation in modern human populations have been used to support the view that the origin of Homo sapiens is the result of a recent dispersal event known as the “Out of Africa” model. • Supporting evidence for this model has come from studies of mitochondrial DNA (mtDNA), which is inherited through the maternal line. • There are fourteen ancestral populations in Africa, but only one outside Africa. • Discuss the pattern of dispersal this evidence supports and why the evidence has been used. • In your answer: • describe the “Out of Africa” model • explain why mtDNA was used to provide evidence for this model • explain, using the “Out of Africa” model, why more diversity is found in the African Homo sapiens populations compared to those of Asia and Europe.
What is mitochondrial DNA? • Mitochondria are structures within cells that convert the energy from food into a form that cells can use. Although most DNA is packaged in chromosomes within the nucleus, mitochondria also have a small amount of their own DNA. This genetic material is known as mitochondrial DNA or mtDNA. In humans, mitochondrial DNA spans about 16,500 DNA building base pairs, representing a small fraction of the total DNA in cells. • Mitochondrial DNA contains 37 genes, all of which are essential for normal mitochondrial function. Thirteen of these genes provide instructions for making enzymes involved in oxidative phosphorylation. Oxidative phosphorylation is a process that uses oxygen and simple sugars to create adenosine triphosphate (ATP), the cell's main energy source. The remaining genes provide instructions for making molecules called transfer RNA (tRNA) and ribosomal RNA (rRNA), which are chemical cousins of DNA. These types of RNA help assemble protein building blocks (amino acids) into functioning proteins
Mitochondrial DNA (mtDNA) is only inherited from ones mother. After 10 generations a person only has mtDNA from 1 person out of 210! • The only source of variation in mtDNA is by mutation.
mtDNA mutates 10 times faster than nuclear DNA so is useful for studying short-term evolutionary change. • By comparing mtDNA they found as expected, closely related people had similar mtDNA.
Therefore an evolutionary tree could be constructed which showed two main branches; the African and everyone else. • This suggests two main groups; those who stayed in Africa and those who left.
By knowing how quickly mtDNA mutates it is possible to date the split at about 200 000ya (or 10 000 generations ago).
mtDNA analysis suggests that we evolved in Africa. • Modern humans are very similar genetically, more so than chimpanzees. This opposes parallel evolution which should produce greater genetic variation.
Genetic and fossil evidence seems to support the replacement hypothesis. • The physical differences between fossil remains of hominin species in Asia vs Africa (and Europe) would have only occurred if they evolved independantly.
Polymorphisms • Polymorphism - Existence of a gene in several allelic forms. • Polymorphic regions provide a very unique set of genetic markers for studying human origin and migratory patterns. • Used to construct a global evolutionary tree of modern man
Mitochondrial DNA • Out-of-Africa hypothesis was first sketched out in 1987, based on mitochondrial DNA analysis • Suggested that modern man first appeared on the scene in eastern Africa about 150,000 years ago, and left between 35,000 and 89,000 years ago, eventually conquering the globe.
Y Chromosomal DNA Study • Researchers looked at DNA samples from 12,000 male Y chromosomes in Asia. • Looking for 3 specific mutations on the Y chromosome known to have originated in Africa. • Researches found that every one of the 12,000 samples carried one of the three mutations or polymorphism
Conclusion to the Y Chromosome Study • Little or no interbreeding of Homo erectus and Homo sapiens. • Individuals are descendants from Africa • Likely that the early African man emigrated to North Africa and made the leap to Asia and then to the rest of the world. • Indicates that modern humans of African origin completely replaced earlier populations in East Asia.
Microsatellite DNA Analysis • Researchers tried to find the estimated time of the deepest split of the human population. • Applied a genetic distance measurement to 30 microsatelite regions to construct a phylogenetic tree for 14 world-wide human populations
What did they find? • In the tree obtained, the deepest root separated Africans from non-Africans. • Their calculations suggest the split happened an en estimated 115,000 to 156,000 years ago.
mtDNA Analysis • Study on the complete mitochondrial genome. • 16,500 base pairs in each sequence • 53 people diverse from different geographical, racial, and linguistic backgrounds.
Results • A tree rooted in Africa • Tree suggests that some Africans are closer to Europeans and Asians than to other Africans.
Conclusion • DNA sequencing evidence shows that modern humans originated in Africa and migrated north out of African, then eventually to the rest of the world. • Oldest fossils of modern humans are found in Africa dating around 160,000 years old.
