The Garden of Eden and Mitochondrial Eve

Satan Watching the Endearments 
of Adam and Eve

William Blake’s etching of Adam and Eve affectionately canoodling while the Devil looks on is one of over 300 works by the artist and poet currently on show at Tate Britain. So deep-rooted is our religious culture that this scene of innocence comes loaded with ideas of guilt from the creation story: disobedience to God, original sin, and so forth.

If you believe in the literal truth of the bible, it is natural to wonder where the Garden of Eden was located, and even if you don’t, it is interesting to ask where on earth Homo Sapiens first evolved. Now Vanessa Hayes. a geneticist at the Garvin Institute of Medical Research in Sydney, and her colleagues believe they have found the answer, with the help of mitochondrial DNA and some recent paleo-geographical research.

Mitochondria are organelles (semi-independent sub-units) in human cells that have many useful functions; most importantly they produce about 90% of the chemical energy the cell needs to stay alive and do its work. They are thought to have once been -- long long ago -- invading bacteria that decided to stick around and seal a symbiotic pact with their host. These freeloading mitochondria have their own modest store of DNA (called mtDNA) that is separate from the main human DNA in the nucleus There are roughly 16,500 letters (nucleotides A, C, G and T) in a mitochondrion’s DNA compared with 3.2 billion in the human genome. However, unlike human DNA, most of which gets scrambled through sexual reproduction in every generation, mtDNA is passed on unchanged from mother to child through her egg. This fact means that it can be traced back indefinitely along the maternal line to form a mitochondrial tree.

At the top of this tree there is a “matrilineal most recent common ancestor” (MRCA) of all living humans; observe that, of necessity, she would have had at least two daughters. because had she had only one, that daughter would be a more recent common ancestor. Journalistic accounts have dubbed her “Mitochondrial Eve” after Adam’s mate in the biblical story of creation that caught Blake’s artistic imagination. Unfortunately this description has led to some misunderstandings. Unlike the Eve in the Book of Genesis, she was in no sense the “first woman”. This is because she is believed to have lived sometime between 100,000 and 200,000 years ago in southern Africa and there were plenty of other women around at the time. Secondly, as matrilineal lines die out, the identity of the MRCA will change and move forward in time. Here is a simple example to illustrate why: imagine that your maternal grandmother had just one sister and that this sister had a just one (now deceased) daughter and via her just one grand-daughter.  While that granddaughter is still alive, your great-grandmother is the matrilineal most recent common ancestor of your family, but if that granddaughter were to die daughterless, then the MRCA role would pass to your maternal grandmother. (There is also a Y-chromosomal Adam. but that is another story.)

The Okavango River Delta

The Makgadikgadi desert and the lush inland delta of the Okavango river now in Northern Botswana were once covered by the largest lake in Africa, 200,000 years ago the fertile wetlands surrounding this lake were teeming with wildlife and offered the perfect nursery for the early development of Homo Sapiens. For the following 70,000 years it is conjectured that these early humans evolved in idyllic isolation, hemmed in by the inhospitable desert-like lands encompassing them. Then, in two bursts of local climate change, fertile corridors opened up first to the south around 130,000 years ago and later to the north 110,000 years ago allowing these people to leave their Garden of Eden and migrate further afield to settle in southern and central Africa.

All genetic information is subject to random mutations and the mitogenome is no exception. It has a certain region of higher variation with roughly 1000 nucleotide letters, and the mutations occurring in this region. together with knowledge of the average rates of mutation, can be used to establish the order in which mutations occurred and thereby to deduce the structure of the mitochondrial tree. As the branches of the tree move back through time, they converge on a group of mitogenomes known as L0, which are characteristic of a group of Khoisan people found in southern Africa who long predate the arrival of the Bantu peoples and Europeans in the area. Professor Hayes and her colleagues assembled a total of 1,217 variants of the LO genome, and from this mtDNA data and knowledge of where it was collected, they were able to construct a more refined mitogenomic tree. This enhanced tree was found to converge to an approximate time of 200,000 years ago and to a location in Northern Botswana. The geographical hypothesis about the isolated fertile region of prehistoric Northern Botswana is supported by research of Axel Timmermann at the Institute for Basic Science in Busan, South Korea. He is a climatologist who has used paleo-geographic and astronomical evidence to deduce the existence of the two periods when the opening of green corridors made migration away from Botswana possible. Thus we have a new creation story, now supported by science, about a Garden of Eden in Botswana and man’s escape -- but not expulsion --  to go forth and multiply and populate the earth.

Other professionals in the field have challenged Professor Hayes’s conclusions as highly speculative, raising doubts about the validity of extrapolating from living mtDNA data into the distant past and about our imperfect knowledge of the migration of peoples over a such a long period as 200,000 years. It has recently been found possible, in rare cases, to retrieve usable genetic information from ancient human remains, and so perhaps older corroborative evidence will be discovered to give stronger credence to this story.