Homo Sapiens Genetics and Diversity

Mitochondrial Eve

• All living humans can trace ancestry to a single African woman, Mitochondrial Eve, who lived ~200,000 years ago (200 kya).
• This conclusion is drawn from Mitochondrial DNA (mtDNA) analysis.
• Mitochondria exist in large numbers within each cell, containing their own DNA.
• mtDNA is distinct from human DNA and is passed down through the maternal line.
• Males are considered "dead ends" in mtDNA lineage tracing because they do not pass it on.
• Regardless of gender, individuals inherit mtDNA from their biological mother.

Mitochondrial DNA Lineage

• The chart depicts the relationship of mtDNA lineages globally.
• Vertical branches represent individuals from major populations.
• Taller vertical distances indicate greater sequence divergence.
• Sequence divergence arises from accumulated mutations.
• African populations exhibit the deepest nodes, indicating longer habitation and greater sequence divergence.
• Populations in Northeastern Africa share mtDNA with those outside Africa, likely due to migration.
• By contrasting mtDNA mutations (represented as lightning bolts), the genetic distance between different populations becomes apparent.
• For instance, comparing an Italian and an Australian sample reveals approximately 10 mutational differences, symbolizing their shared common female ancestor.
• Comparisons between two African samples, like the Kikuyu and San Bushman, often show a larger number of mutations, such as 21. This difference highlights the longer evolutionary history within Africa. I/A_{CA}
• The speaker showed another chart that had identical evidence as the first one, but flipped upside down and mirrored.
• The speaker referenced a paper that shows how using mitochondrial DNA affects human migration.

Y Chromosome Tracing

• While mtDNA traces the female lineage, the Y chromosome is used to trace the male lineage.
• A significant portion of the Y chromosome doesn't pair with the X chromosome, allowing for tracing.
• Analysis of the Y chromosome reveals a similar pattern to mtDNA, with the most divergence in African samples.
• Y chromosome evidence supports the "Out of Africa" theory, aligning with mtDNA findings.
• Y chromosome DNA dates back 70,000 years.

Modern Human Populations

• Mitochondrial DNA and Y chromosome DNA confirm that modern human populations are genetically close and diverged recently from a common ancestor.
• A bottlenecking effect likely occurred as humans left Africa.
• Homo sapiens replaced Neanderthals through demographic advantage and assimilation.
• Modern humans show less gene diversity than local ape populations.

Genetic Diversity Comparison

• Neanderthals are genetically closer to humans than chimpanzee populations are to each other.
• A chart compares mitochondrial diversity among ape species, including bonobos, chimpanzees, humans, gorillas, and orangutans.
• Humans have a small genetic cluster, indicating limited diversity.
• Early mtDNA data suggested no interbreeding between Neanderthals and humans.
• Nuclear DNA evidence now confirms interbreeding occurred.

Human Genetic Variation

• Human genetic variation is comparable to that of eastern chimpanzees.
• Human diversity resembles a subpopulation within Africa that expanded globally.
• Humans are incredibly nondiverse based on mtDNA.

Chromosomal DNA

• Genetic diversity within hominoid populations reveals that humans have less total genetic diversity as a species.
• Local human populations maintain a high percentage (86%) of total genetic diversity.
• Humans are highly inbred, sharing 99.9% of genetic material.

Human Migration and Intermixing

• Humans migrated across the globe in waves, reaching Australia by 30,000 years ago and the Americas 10-13,000 years ago.
• Subsequent mixing has overwhelmed local genetic oddities.
• Global travel facilitates intermixing, unlike other species.

Skin Pigmentation and Race

• Race is a relevant issue, often categorized by skin color.
• Skin pigmentation is a superficial genetic trait.
• Historically, skin pigmentation was wrongly used as a proxy for biological information.
• Skin pigmentation varies clinally with distance from the equator.
• Melanin protects against UV radiation, acting as a natural sunblock.

MC1R Gene

• MC1R, a gene on chromosome 16, relates to skin and eye pigmentation.
• Skin pigmentation is controlled by multiple genes.
• As one moves further away from the equator, there is less of a need for dark skin.
• Natural selection is more prevalent closer to the equator.
• There are few MC1R gene variants within African populations, and all preserve function.
• Outside of Africa, there are more mutations, with many resulting in loss of function, leading to lighter skin.
• Natural selection is more relevant closer to the equator for darker skin.
• Genetic drift explains MC1R mutations outside Africa.

Race as a Social Construct

• Race is not strongly correlated with genetics.
• Races were developed to categorize people, but they correlate with biology.
• Most genetic diversity is within populations (86%).
• Population differences are clinal and influenced by environment.
• Race is a social construct with no real biological linkage.
• There are some environmental genetic differences in populations due to their environment such as red blood count in high altitude populations.
• Race, based on a social setting, has negative impacts that have been made in the past.