Chapter 23.2: African Origins

Hypothesis for Human Origins:

1. Multiregional Hypothesis: 

2. Out-of Africa Hypothesis

Multiregional Hypothesis:

• Proposed that modern humans evolved independently in multiple regions from Homo ergaster populations that left Africa ~2 million years ago

  • Connected through minimal gene flow → allowed it to maintain as single species

  • Therefore, humans from different regions have different ancient roots

• Predicts: 

  • Regional populations gradually evolved into modern humans over 2 Ma

  • Racial differences among humans reflect local adaptations that developed over a very long period of time

• REJECTED: Due to the mtDNA test which proved we all evolved from africa

Out-of Africa Hypothesis

• Suggests that modern humans evolved in Africa from Homo ergaster descendants (sometimes called Homo heidelbergensis) ~200,000 years ago

  • Claim: All modern humans came for this african population 

    • Greater genetic diversity in Africa due to a longer evolutionary history 

• We then migrated out of Africa and replaced other earlier hominins → and evolved different traits slowly 

• Molecular Evidence: Rebecca Cann’s Mitochondrial DNA Study

• Rebecca Cann’s Mitochondrial DNA Study

• Background: Compared mtDNA sequences from 147 people around the world

  • This approach allowed reconstruction of the human family tree

• mtDNA Use: Ideal for tracing ancestry and evolutionary relationships 

  • Small (~17,000 base pairs) and circular

  • Abundant: many copies per cell

  • Maternally inherited - from the egg; sperm do not contribute mitochondria

  • Non-recombining,ng

• Method:

  • Collected mtDNA from placentas at birth 

  • Used restriction enzymes to cut the DNA at specific recognition sites → IF a mutation changes the sequences the enzyme does not cut 

    • Formed fragment patters in gel electrophoresis that showed sequence differences 

• Results: 

  • Deepest Branches: African mtDNA 

    • modern humans originated in Africa, and non-Africans are descendants of a subset of Africans that migrated out

    • Non african mtDNA had branched from within the african tree

  • Shallow Tree

    • Even the most distantly related humans have a recent common ancestor (~200,000 years ago)

• Extension studies: Y chromosome that do not recombine - in men

  • Genetic Drift Effect

Non-recombining

• meaning mutations are the only source of variation → clear lineage tracking

Phylogenetic Reconstruction

• USed shared derived characters to reconstruct the mtDNA family tree 

  • Mutations that arose in common ancestors that are shared by certain people

Genetic Drift Effect

• the random change in the frequency of gene variants in a population over time, leading to reduced genetic variation 

Neanderthals

• Biological species: If we could interbreed then they were considered the same species as us….

• DNA Studies: 

  • mtDNA was extracted from neanderthal bones 

    • No close matches between humans and neanderthals - No interbreeding

  • Whole Genome Sequencing

    • Interbreeding did occur between neanderthals and early non-african modern humans 

    • 1–2% of the DNA in all non-African humans is of Neanderthal origin

Questions on whether humans killed them or if we just interbreed with them

Denisovans

 ANother group of archaic humans - discovered in the caves of siberia

• Related to neanderthals and interbreed with some modern human populations 

  • Specifically the australasians → they have denisovan DNA in them 

Functional Importance from archaic Genes: Tibetan High-Altitude Adaptions

• EPAS1 gene - helps regulate oxyygen metabolism at low O₂ levels

  • The Tibetan allele is similar to Denisovan EPAS1, suggesting introgression 

Introgression

•  the transfer of genetic information from one species to another as a result of hybridization between them and repeated backcrossing