Anatomists have studied the development of embryos for centuries. The development of our fetus almost appears to follow the evolution of modern humans - from fish to mammal to primate.
However, for a better history of our development, the place to look is in our DNA. For those that understand genetics, DNA reads like a Tolstoy novel complete with a wide range of characters and several thousand pages of text.
The amount of DNA in a single cell is immense. DNA is made up of atoms which are incredibly small. A hundred million atoms stretch across the period at the end of this sentence. Lining up 10 billion atoms in a row results in a line one centimeter long.
If you line up enough atoms in a row, you can get a molecule with measureable length and DNA is such a molecule. The total DNA in a single cell measures about two metres long so each partner in a chromosome measures slightly under five centimetres in length.
There are billions of base pairs in each strand of DNA and many atoms involved in each base pair.
A single strand of DNA has a backbone roughly five billion atoms long.
The billions of base pairs in our DNA contain all of the information needed to create a whole human being. But they also contain a history of our evolution from single-celled organisms to the complex modern creatures we are. Indeed, our DNA contains information from before we were even single-celled organisms.
Human cells are filled with organelles. One of them is the mitochondria, the cell's powerhouse. It houses the apparatus which allows us to gain energy from sugar. Each cell in the human body is littered with mitochondria.
In 1963, two separate teams working independently discovered mitochondria have their own DNA.
At first, biologists hypothesized this was just a happy accident.
Biologists generally accepted the nucleus was the only source of all DNA. They assumed at some point in the distant past, the nucleus loaned out a small portion of its DNA to the mitochondria and just never got it back.
It was the maverick biologist Lynn Margulis who pushed the idea that life had more ways of mixing than conventional biologists imagined. She envisioned the process of endosymbiosis.
Life is descended from the same early microbe - the Last Universal Common Ancestor. Indeed, all life has around 350 genes in common - a silent testimony to our common ancestor. These genes govern some of our most fundamental biochemical processes.
Early life was subject to environmental pressure and soon enough microbes started to diverge. Some grew to enormous size. Other shrank to tiny specks. Each was best adapted to its own niche in the microbial world.
Not surprisingly, perhaps, some microbes determined other microbes were a good source of food. The large began to eat the small and the small fought back by infecting and killing the large and unwary.
Margulis's argument is that one day, in the early history of life, a large microbe ate a small one and instead of digesting it the two organisms developed a mutually beneficial relationship. It likely took generations but eventually this hostile encounter became a cooperative venture. Gradually, the little microbe lost its ability to live as a separate organism but it became very good at generating high-energy cellular fuel.
The larger cell lost the ability to synthesize its own energy but it was otherwise a great host for its symbiotic partner.
The division of labour was of mutual benefit and the longest relationship in the history of life began.
Proving all of this took Margulis a long time. There was considerable resistance to the idea all life was a composite or the different organelles in a cell might have resulted from endosymbiosis.
However, over time, the evidence became insurmountable.
The clinching argument was mitochondrial DNA.
We inherit this DNA differently than chromosomal DNA. It is passed exclusively from mother to child. It tells the story of our matrilineal descent. It contains the history of our mother and our mother's mother and so on back through time.
Mitochondrial DNA opened up a whole new area of science in the form of genetic archaeology.
Mitochondrial DNA is both more abundant in cells than chromosomal DNA and it is also more conserved. It can be used to trace genealogies with unprecedented accuracy as changes in mitochondrial DNA occur at the rate of one mutation every 3,500 years.
In fact, this clock tells us all
7.5 billion people alive descend from a single woman who lived in Africa about 170,000 years ago.
She has been dubbed Mitochondrial Eve and, although she wasn't the only woman alive at the time, she is our oldest common matrilineal ancestor.
The history of the human race is written in our DNA. We are all related.
