- Allele diversity: With this method, scientists measure how many alleles (gene variants) are found in present-day humans. They then estimate the ancestral population size that would be needed to produce all those alleles given the human mutation rate and the mathematical probability of new mutations spreading in a population or being lost.
- Linkage disequilibrium: This method takes advantage of the fact that there is a well-characterized relationship between (a) the distance between two genes on a chromosome, and (b) the likelihood that “crossing over” (a process of chromosome breakage and rejoining) will occur between those genes. If two genes are located close to each other on the same chromosome, the alleles present at both locations tend to be inherited together (i.e., a crossing over event is unlikely to occur between them). So with this method, scientists look at the allele combinations that are found in present-day humans, and then they estimate the ancestral population size that would be needed to produce those combinations given the crossing-over frequency.
- Incomplete lineage sorting: This method exploits the fact that we expect the relatedness pattern of certain genes to sit at odds with what we expect on the basis of species relatedness. For example, although humans and chimpanzees are the closest living relatives of each other as species, we expect that some human genes will be closer matches to those of other great apes, such as gorillas. This allows scientists to infer what genetic variants were present in the common ancestral populations, which allows them to estimate population size.
Thus, there are multiple, independent, converging lines of evidence that support and substantiate the conclusion that humanity began with a population of several thousand people.
Evolution is a population-level phenomenon
The idea that humans descend from several thousand individuals is really a corollary of evolution. Evolution is a population-level phenomenon; speciation takes place by incremental changes of average characteristics after two populations separate. To make this idea more understandable, Venema invokes a helpful analogy: how languages change over time.
Changes within a language take place over long timescales
and within a population of speakers. Venema compares a modern translation of
John 14:6 with the way it was translated in Anglo-Saxon over a thousand years
ago. The two are barely recognizable. Despite the striking differences we see
now, however, the process that produced them was gradual. Anglo-Saxon
incrementally became Modern English over generations, within a continuous
population of speakers. Each generation could easily understand their parents
and their offspring. Over time, however, changes accumulated that gradually
shifted the language. There is no convenient point where we can say Anglo-Saxon
“became” Modern English; the process was a continuum. Venema demonstrates this
by providing a sampling of English translations of John 14:6 from the Middle
Ages to the present.
Like change within a language, genetic changes within a population are incremental, and every generation remains the “same species” as their parents and offspring. Over time, however, genetic changes can accumulate to the point where generations far removed from each other would not reasonably be considered the same species.
The ability of the characteristics of a language or genome to incrementally shift over time also means that an ancestral population (of speakers, or of organisms) might give rise to two or more distinct populations over time. Again, Venema invokes an analogy to language. Venema has ancestors that spoke West Frisian, a language most commonly found in Friesland (a province of the Netherlands). English and West Frisian share a common ancestral population of speakers that lived in what is present-day northern Germany and Denmark around AD 400. A group from this population crossed the English Channel and settled in England, with the rest of the group remaining on the European continent. Once these two populations separated, changes in how they spoke their (once identical) language began to accumulate. Since the populations were geographically separated, the changes were independent of one another; the shifts in one group were not the same in the other. Over time, the two populations became more and more distinct. Moreover, there is no convenient point at which they “became different languages.” Each generation in both populations spoke the “same language” as their parents and their children; the process by which the languages became distinct was incremental.
Similarly, if a population is separated into two groups, the incremental process of allele shifting over time is independent in the two groups. Thus the average characteristics of the two populations may now begin to diverge from each other. Given enough time, the characteristics of the two populations may become different enough that, even if the two groups were brought into contact with each other again, they would not recognize each other as members of the same species. Even though each generation in both populations is the “same species” as its parents and its offspring, gradual shifts in characteristics can, over long timescales, produce distinct species.
The bottom line is that humans did not just suddenly appear. We evolved as a population.
What about the story of Adam and Eve?
Both Venema and his co-author, Scot McKnight (a New Testament scholar and a Christian), offer some helpful perspectives for Christians who take the scientific evidence seriously.
Venema reminds us that science is unable to weigh in on whether Adam and Eve were real people. Science tells us that if Adam and Eve were in fact historical, they were not the sole parents of all humanity but part of a larger population. Beyond this, science cannot say.
In the second part of Adam and the Genome, McKnight seeks to explain Adam and Eve as they were intended to be understood in the ancient world. He makes several points that are worth considering:
—The first few chapters of Genesis were produced in the
ancient Near East, and they were designed for the ancient Near Eastern culture.
They use categories, terms, and ideas from that world, and they share the
pre-scientific assumptions of that world. There are similarities between
Genesis 1–11 and other ancient Near Eastern texts (like Enuma Elish, the
Gilgamesh Epic, and Atrahasis), but there are also profoundly different
theological ideas.
—Precisely how Genesis’s depiction of Adam and Eve relates to history is not something easily knowable from the text of Genesis itself, so characterizing Genesis as pure myth goes beyond what we know. At the same time, mythical themes and language do have some role in the Bible’s creation accounts.
—Many of the things that modern Christians teach about Adam and Eve are not found in Genesis or even in Paul’s writings in the New Testament. For example, Paul does not say that all humans have a biological relationship to Adam and Eve, or that Adam and Eve passed on their sinful natures to all humans.
Questions I’m pondering
To what extent should the story of Adam and Eve (as found not only in Genesis, but also in Mormon scripture such as the Book of Moses) be taken literally?
To what extent is it important to Mormonism that Adam and Eve were real people?
To what extent is the idea that all human beings descend from Adam and Eve important to Mormonism?
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