[#39] Naming Your Ancestors

[#39] Naming Your Ancestors

By Piet Hut

In a very real way, you are much older than any of the rocks in any of the mountains you will ever encounter.  Each living cell in your body is directly connected in an unbroken chain from LUCA, the Last Universal Common Ancestor, that lived about four billion years ago.

When you give your age, you count from the day you were born. But you were alive well before that time.  And even at the moment of conception, the sperm and egg cells of your father and mother were alive and well before they met.  They in turn were produced by cell division tracing back to the moment that each of your parents were conceived, and so on.

The tree of life, as sketched by Ernst Haeckel (1834-1919)

The amazing thing is that each of us can trace the life of each of our cells directly back to LUCA, the cell (or more realistically the community of cells) that lived at the very root of the wide variety of forms of life on our planet.  All living cells that ever have been found in any living organism share a common pattern of DNA and a common basic set of metabolic processes, and this can only be explained by a common origin, situated about four billion years ago, half a billion years after the formation of our planet.

You are the product of an incredibly complex network of ancestors.  And each of those ancestors managed to procreate before they died.  If any of them had died before passing on their genes, your genealogy would be different, and most likely you would be different.

How many ancestors are we talking about?  For starters, let us estimate how many generations separate you from LUCA.  In other words, pick one of your parents, then pick one of the parents of that parent, and so on.  One example of selecting one such lineage is choosing the father at each step, as is done for last names in our society.  Another example is choosing the mother, as in doing in the Jewish tradition to determine who is Jewish.

To make an estimate of the number of generations, we can focus on unicellular organisms, which dominated the Earth during at least the first three billion years of the four billion years that life has been around.  They are present in any line of ancestors we choose, and in far greater number than the later multicellular animals that eventually contributed their genes to the human species.

The reason for this larger number is twofold: not only have animals been around for a much shorter time than simple single-cell organisms, also the lifetime for a single cell to divide is much shorter than the lifetime of a multicellular organism.  Together this means that we can completely neglect the number of generations in ons lineage that came after the Cambrian explosion.

Under favorable conditions, a single cell can divide in much less than an hour.  But let us take a modest approach, by guessing that on average most of our early ancestors divided once a day.  In the first three billion years, there were about one trillion days, so a good guess for what separates us from LUCA is somewhere around a trillion generations.

Now imagine that you wanted to name your ancestors, just only one in each generation.  This would be like the old system of Mary, the daughter of Charles, who is the son of John (which is how the last name Johnson came into being), etc.  A typical large novel contains about one hundred thousand words.  So you would need to print ten million such books, in order to publish the names of a single lineage of your ancestors.

Conveniently, this is about the number of books in the Library of Congress of the United States, the largest library in the world.

The Thomas Jefferson Building, one of several buildings containing the Library of Congress

This gives an impression of the stunning complexity of the process that gave rise to the appearance of each of us.  Even a single path, any path, through our complex interwoven genetic history already can fill the largest library on Earth, if each ancestor on that path was named!

And if we were to name each of the ancestors in the complete network that gave rise to us, we would wind up with a fair fraction of the full history of the biosphere, especially if we were to take into account horizontal gene transfer, as an important additional mode of connecting branches in the tree of life, in addition to the vertical connections provided by cell division and sexual reproduction.

A modern tree of life, with horizontal gene transfer

A conservative estimate would be to count the number of cells present today in the biosphere, which has a mass of more than a trillion tonnes.  With a typical cell being smaller than 10 micron, and therefore weighing less than a billionth of a gram, an estimate of the number of living cells on Earth today thus gives an order of magnitude of a trillion times a million times a billion, or 10 to the power 27.

Printing that many names would produce a number of books of 10 to the power 22, ten sextillion (ten billion trillion) volumes.  Another simple calculation shows that not only the whole surface of the Earth would be densely covered by books, but that the books would pile up to the height of Mount Everest, covering all continents stacked closely side by side, and burying all mountains under them.

And this is still likely to be an underestimate of the number of books needed, if you wanted to give a short name to each of your ancestors, and print out all those names.

Piet Hut is President of YHouse (where this blog is hosted), Professor of Astrophysics and Head of the Program in Interdisciplinary Studies at the Institute for Advanced Study in Princeton, and a Principal Investigator and Councilor of the Earth-Life Science Institute in the Tokyo Institute of Technology.