An article published in the journal “Nature” reports the results of an analysis of modern humans’ mitochondrial DNA to reconstruct the Homo sapiens family tree. A team of researchers led by Eva Chan of the Genomics and Epigenetics Division of the Garvan Institute of Medical Research and the University of New South Wales made a complex genetic analysis concluding that the modern human species emerged about 200,000 years ago in today’s Botswana, near the Zambezi River.
For some time Africa has been considered humanity’s birthplace but there are many discussions concerning the exact place where the first individuals with Homo sapiens characteristics were born, important also to try to understand the relationships with other hominins. Various researches have been carried out on fossils found over time but it’s difficult to obtain comprehensive answers from limited amounts of bones.
For this reason, the team led by Eva Chan chose an approach based on the analysis of mitochondrial DNA, the one present in the organelles of eukaryotic cells called mitochondria and separated from the DNA of the cell nucleus. This DNA, also known as mitogenome, is transmitted from mother to child and it’s possible to reconstruct the human family tree through its mutations.
The researchers combined 198 new and rare mitogenomes and a database of the so-called L0 lineage, the oldest known human mitochondrial lineage for a total of 1,217 mitogenomes. The reconstruction of the human family tree was based on mutations of mitogenomes and on other elements connected to language, culture and geographical distribution of different sub-lineages. The result is that the first anatomically modern Homo sapiens emerged about 200,000 years ago in an area south of the Zambezi river basin, which includes today’s northern Botswana, expanding to Namibia to the west and Zimbabwe to the east.
The area that may have hosted humanity’s birthplace also held the ancient Lake Makgadikgadi, which had started draining due to shifts in the tectonic plates before modern humans emerged. For a very long time that was a wetland with optimal conditions for the prosperity of an ecosystem. Long-term stability may have allowed modern humans to thrive as well for a full 70,000 years.
The image (Courtesy Chan et al., Nature 2019. All rights reserved) shows on the left a map of the distribution of ancestral DNA among the populations used as a sample. The result is the identification of what the researchers claim is the region, shown in light orange on the map on the right, in northern Botswana that might be humanity’s birthplace.
This study also reconstructed the first possible migrations from that area thanks to the oldest genetic divergences. According to the researchers, those migrations took place between 130,000 and 110,000 years ago, first to the northeast and then to the southwest with various adaptations to different climates.
This study was based on mitogenome analysis but also used other scientific disciplines to obtain more precise results. Despite this, the results were received with caution due to the difficulty of accurately reconstructing such ancient events. Some researches proposed a more complex origin of the Homo sapiens species, with migrations within Africa of different ancestral populations that subsequently mixed up again into what we consider modern human beings. Genetic traces may be insufficient to reconstruct those events, at least for now.
The discussions will continue, also because it’s difficult to define exactly what a Homo sapiens is. Two articles published in June 2017 in the journal “Nature” described different aspects of a study on various fossil bones including a skull and a jaw uncovered at a site in Jebel Irhoud, Morocco, dating back about 300,000 years ago and attributed to Homo sapiens. It wasn’t possible to derive DNA from those fossils so the evaluations are based on anatomical characteristics. This shows the complexity of this type of study and advances in genetic techniques can help.