In this series, we explore the genetic evidence that indicates humans became a separate species as a substantial population, rather than descending uniquely from an ancestral pair.
In yesterday’s post, we drew an extended analogy between genetic change within a population over time, and change within a language over time. While no analogy is perfect, this one is remarkably good – and it will continue to be useful as we now turn to discussing how new species form.Flags of Friesland and England
With a last name like Venema, it will come as no surprise to many that I have Frisian ancestors. Friesland is a province of the Netherlands with a distinct language, West Frisian, that is one of the most closely-related modern languages to English. My paternal grandparents, immigrants to Canada in the post-World War II era, spoke a delightful hodge-podge of English, Dutch and Frisian to each other, often using mostly Frisian and only reverting to Dutch or English if it happened to have a better word for the concept at hand. As a child, I remember hearing Frisian and noting how similar many of its words were to their English equivalents – far too many similarities, I thought, to be merely coincidence. A well-known sentence will serve as an illustration: it is pronounced essentially the same in both Frisian and English:
Butter, bread, and green cheese is good English and good Frise.
Bûter, brea, en griene tsiis is goed Ingelsk en goed Frysk.
Later, I would come to understand the reason for these striking similarities – English and West Frisian are modern descendants of an ancient language – they share a common ancestral population of speakers. As we have seen, languages change over time. In the case of English and West Frisian, the original population, which spoke a language ancestral to both modern-day English and modern-day West Frisian, was divided: some remained on the European continent, and others travelled to what is present-day England. Once separated, each sub-group – which at the point of separation spoke the same language – went on to acquire differences over time that were independent of each other. In due time, the changes that accumulated in both groups rendered them mutually unintelligible to each other: they had become separate languages. The precise point at which they became “different languages” of course is impossible to pin down, since there was no such precise point when it occurred. Both groups changed their language over time incrementally, as a process on a gradient.
Species form through a process much like language development. Should two populations of the same species become separated, they too can accumulate genetic changes that shift their average characteristics over time – changes that are not shared and averaged across the two groups due to their genetic isolation. In many cases, geographic isolation acts as the first genetic barrier – much like the ancestors of the English parting ways with their continental cousins. Once separated, should enough genetic changes accrue between the two populations, eventually they will not recognize each other as members of the same species – which would be analogous to my experience as a modern English speaker in present-day Friesland.
Species, like languages “begin” as populations – and “begin” isn’t the right word
Our language analogy also helps to counter some common misconceptions about how species form. Often when I am speaking about human origins, folks who have heard about modern humans descending from around 10,000 ancestors wonder how on earth those 10,000 people suddenly appeared on the scene without ancestors. Of course, they didn’t suddenly appear – they too have a population that gave rise to them, and so on – stretching back to our shared ancestral populations with other apes and beyond. There is in fact no point in our evolutionary history over the last 18 million years or more where our lineage was reduced below 10,000 individuals – and 18 million years ago our lineage was not even close to “human” in any sense, since this time predates any hominin (i.e. species more closely related to us than to chimpanzees) in the fossil record. The 10,000 number is simply the smallest population size we have in our history over the last several million years. The lineage leading to modern humans experienced what is known as a population bottleneck, a time when our ancestral population size was reduced to around 10,000 breeding individuals, only to expand again afterwards.
Despite these explanations, it is a common misperception to think that species, should one go back far enough, started off with one ancestral breeding pair “becoming different”. Hopefully our language analogy is useful here. It is of course not reasonable to expect that English or West Frisian got its start when two individuals began speaking a new language. The lineage that led to either modern language did so as a population of speakers – and each generation within both lineages was perfectly intelligible to their parents and their offspring. In the same way, species form as populations shift their average characteristics over time – but always remaining the “same species” as their parents and their offspring. So, to speak of a language or a species “beginning” is to hit up against the limits of language. Neither “begins” in some sort of discontinuous sense – they become, over time, in continuity with what came before.
So, one of the reasons for the common expectation that humans had a discontinuous beginning with a single ancestral couple has nothing to do with Genesis or Judeo-Christian theology: I have met many non-theists who also have this same expectation. We are used to thinking in discontinuous terms – species should have a defined beginning – and we are used to thinking that species therefore begin with a radical change to a single ancestral pair. For evangelical Christians, these incorrect assumptions about how species get their start fit hand in glove with the common view that since species require a discontinuous start, only God can miraculously provide it through an event of special creation. Once one understands how species form as populations over time, however, one is then prepared to investigate the question of how large our population was as we became human – a question we will begin to address in the next post.