A chicken is an eggs way of making another egg
One of the joys of Ontology building is that you can end up in some fairly obscure arguments; the one I got in today is whether a sperm is a human being. Of course, this is silly, but mostly because of the limitation of our language. I would like to describe here why a sperm is a human individual and why it is important.
One of the long running discussions in the ontology community is how we define function. With respect to biological organisms and biological function this is particularly challenging; in fact, biology continually raises questions and exceptions which is part of the fun.
I added my contribution to definitions of function several years ago (n.d.) built largely around evolution and, more importantly, homology.
One of the issues with other definitions available at the time, and specifically, BFO is that it used a definition as follows:
A biological function is a function which inheres in an independent continuant that is i) part of an organism[…]
The point, here, is that by definition an organism cannot have a function because an organism cannot be part of an organism. This works well for people, but badly for some organisms, especially eusocial ones like ants which appear to have functions in their society which they have evolved to fulfil. My argument here is that it also means that a sperm cannot have an function, because, actually, a sperm is an organism. Of course, this seems daft; a sperm is, surely, part of an organism in the same way that a blood cell is. However, this is not true.
All organisms have a genome — their genetic material. Many organisms have a single copy of their genome; for single celled organisms, this gets doubled before they divide, so actually they have two copies of their genome much of the time, but these two copies are identical. These organisms are called haploid.
However, as you might expect sexual reproduction makes things more complex. This involves taking two previously independent organisms, merging them, then dividing again. The merged organism has, now, two different copies of genome; these are called diploid.
Once this happens, the life cycles of an organism gets more complex. Some organisms, such as the yeast (Schizosaccharomyes pombe) really dislike being diploid. It’s possible to maintain them in the lab, but generally given the choice they sporulate and become haploid again. However, others such as brewers yeast (Saccharomyces cerevisiae) behave differently. It grows, develops and lives as in the haploid form; but it also does this in the diploid form and is quite happy.
Many plants do this also and exist in both a multicellular haploid form (called the gametophyte) and a multicellular diploid form called the sporophyte. In the flowering plants, the gametophyte is very small, and exists entirely within the sporophyte stage; in other plants, the gametophyte is larger. But both forms can grow and develop, a process called alternation of generations.
As far as I know, no animals do this. However, there are quite a few organisms where both a haploid and a diploid form exists; the male ant that I refered to earlier will be a haploid, while the females are diploid. This doesn’t disadvantage the male — it simple produces sperm which are genetic clones of itself.
In humans, like the flowering plants, the diploid form is dominant. There are two haploid forms, the egg and sperm, both single cells; the female form exists entirely within the diploid from which it arose; the sperm can travel a bit further but not much.
Of course, in most practical circumstances, the sperm would appear to be a part of the man that produced them; if I was building a medical ontology, I would make this statement, because it would fulfil everyones intuition, common medical and legal practice.
But, there is no real justification for this. It exists, it is independent from that man, has a different genome from that man; it is an organism in the same way that a gametophyte or a male ant is an independent organism. For a biological ontology, working cross-species, we have no basis for making this distinction; if the sperm has a function of fertilizing an egg, then the man has the function of producing more sperm. Alternatively, if a man cannot have a function, neither can a sperm.
Does this mean that sperm is a human being? Obviously this would be silly, nor is a sperm a person; but it is an organism and it is human. We just lack a word to describe this.