There are over 7 billion people living on Earth – all having a few common ancestors who lived tens of thousands of years ago. The exact time and method of how those ancestors gave rise to such a diverse population, however, is unclear.
Anthropologists see Y-chromosomal Adam, and Mitochondrial Eve, as the parents of humanity, who passed down part of their genes to us. Earlier research said that Adam lived much later than Eve, but according to a new study by G. David Poznik et al., they could have lived around the same time.
This is not to say, however, that Adam and Eve were partners, but that these individuals passed on parts of their DNA (Y-chromosome and mitochondrial genome) to the majority of us. The same sequences from others, in comparison, have mostly died out due to natural selection and similar means.
Why the Y-chromosome and Mitochondrial Genome?
The reason for the use of the Y chromosome and mitochondrial genome for many of these ancestral studies is because of the way they pass through generations. We pass the Y-chromosome only from father to son, while we pass the mitochondrial genome, which is less specific, from mother to both sons and daughters.
These genes are special because, unlike other chromosomes, they do not undergo any swapping, or recombining, of genetic material.
Mutations accumulate over time on a haplotype, which is a combination of DNA sequences at adjacent locations on a chromosome that are passed down together, so haplotypes like the male-specific region of the Y-chromosome provide a good inference point for evolutionary history.
Human chromosome pairs swap genetic materials between each other so that it lessens the rate of bad genetic mutations, but since the Y-chromosome does not have a partner, it faces a lot of mutations that can be traced back through time. Researchers estimate that a man’s Y-chromosome differs from his father’s by about 600 DNA letters, thousands of times more than the normal mutation rate.
Genomes and Generations: The Study
Poznik et al. sequenced the genomes of 69 males from many different populations: Namibia, Democratic Republic of Congo, Gabon, Algeria, Pakistan, Cambodia, Siberia, and Mexico.
The researchers identified over 11,000 variants in the sequences that allowed them to more accurately establish phylogenetic relationships and timelines. This allowed them to resolve a polytomy (a section in in the phylogenetic tree where evolutionary relationships cannot be fully resolved due to too many unclear divisions) in the Y-chromosome tree and better show ancestral relationships.
They also identified lineages, from the samples of people from Namibia, that appeared to form just after the root of the Y-chromosome tree. This contrasted earlier findings of the African tree branch being shorter than others.
To compare the Y-chromosome genome to the mitochondrial genome, Poznik et al. estimated their respective mutation rates by using phylogeographic patterns, or genetic patterns seen from geographic distributions, from a well known event – the settlement of the Americas 15,000 years ago.
The researchers compared the male samples with an additional 24 females samples and found that Y-chromosomal Adam to be about 138,000 (120,000 to 156,000) years old and mitochondrial Eve to be about 124,000 (99,000 to 148,000) years old.
DNA Sequencing: Differences Between This Study and Others?
Only recently has it been possible to sequence the Y-chromosome. Previous estimates of the Y-chromosome used other methods, like short resequenced segments and SNPs from a small amount of individuals, that underestimated its genetic diversity.
Previous research dated Adam to be between 50,000 to 115,000 years old and Eve to be between 150,000 to 240,000 years old.
Another study published the same time as Poznik’s puts the Y-chromosome even further back in time to 180,000 to 200,000 years ago. This study was conducted by Paolo Francalacci et al. and used 1204 men from Sardinia. These researchers constructed a tree with all the main haplogroup found in Europe, along with their sample, and calibrated it with the initial expansion of Sardinia.
Adam, Eve, and Human Lineage
Previous studies have said that Adam and Eve lived at different times, with Adam living more recently than Eve. However, the study by David Poznik et al. concludes that this was not so, that Adam and Eve could have lived during the same time, and that Adam could even be a bit older, as suggested by Francalacci et al.
National Human Genome Research Institute. All About the Human Genome Project. (2013) Accessed August 21, 2013.
US Census Bureau. US and World Population Clock. (2013) Accessed August 21, 2013.
Poznik, David G. et al. Sequencing Y Chromosomes Resolves Discrepancy in Time to Common Ancestor of Males Versus Females. (2013). Science. Accessed August 21, 2013.
Rozen, Steve. et al. Abundant Gene Conversion Between Arms of Palidromes in Human and Ape Y Chromosome. (2003). Nature. Accessed August 21, 2013.
Francalacci, Paolo. et al. Low-pass DNA sequencing of 1200 Sardinians reconstructs European Y-chromosome phylogeny. (2013). Science. Accessed August 21, 2013.© Copyright 2013 Thu Anh Le, All rights Reserved. Written For: Decoded Science