This blog was first posted at Nature Ecology & Evolution Community on 4 March 2020
That a single couple could be the ancestors of all living humans is widely seen as an area of conflict between genetics and the Abrahamic religions. Though little detailed attention has been paid to this idea in the scientific literature (see ‘Adam and Eve: a tested hypothesis?’), current models of the history of genomic variation in African populations tend to forbid a bottleneck of two in the human lineage within the last five hundred thousand years (see ‘Adam and Eve: lessons learned’). Thus, belief in a literal pair of ancestors for all humans would entail an older date for Adam and Eve than believers had expected, or a revised understanding of human molecular evolution.
In a recent book, The Genealogical Adam and Eve, S. Joshua Swamidass, an Associate Professor at Washington University in St Louis, Missouri, seeks to resolve the dilemma faced by believers. He makes a major contribution to the debate, taking both sides seriously. Swamidass is a Christian himself, and a well-established scientist in chemical bioinformatics and drug metabolism. He has clearly read widely and deeply in evolutionary genetics. He outlines his ideas with caution and many references to the literature. He deserves a hearing by anyone who is interested in a better relationship between science and religion.
Swamidass suggests that Adam and Eve were a real pair who are present in the genealogical ancestry – but not necessarily the genetic ancestry – of all present-day humans. He draws on simulations published in Nature in 2004 by Douglas Rohde, Steve Olson and Joseph Chang showing that just a few thousand years ago many individuals must have existed who are genealogical ancestors of all present-day humans. Swamidass makes the simple suggestion that one pair of the shared ancestors of all living humans was “Adam and Eve”.
He also points out that not all of our ancestors contribute to our genomes. Those that do not are known as “ghost” ancestors. Thus, Swamidass’s “Adam and Eve” will have contributed nothing to the genomes of some, many, or perhaps even all, living humans. “Adam and Eve” are entirely untraceable using genetic information. Thus, believers can say all humans are descended from Adam and Eve, and no genetic evidence can falsify or confirm that belief.
From a purely genetical perspective, it seems hard to contradict this thesis. The findings of Rohde et al (2004) have been scrutinized and broadly confirmed by several papers that Swamidass cites and explains in his book. The fact that not all of our ancestors contribute to our genomes a simple corollary of Mendelian genetics. Swamidass notes that this view of Adam and Eve is (superficially at least) compatible with a diverse range of possible beliefs about when they lived, where they lived, and whether or not divine intervention was involved in their origin.
Swamidass argues that if his hypothesis is true, then there is “no evidence for or against Adam and Eve, ancestors of us all” (p. 81). Thus, he has an essentially unfalsifiable hypothesis regarding Adam and Eve. By doing this, he reaches something close to Stephen Jay Gould’s non-overlapping magisteria (NOMA) between science and religion, in an area where they have often been seen as making competing claims about reality. He hopes that this may be a way of making peace.
A more detailed scenario
Having established this basic claim that “Adam and Eve” are an undetectable pair hidden among our many shared ancestors, Swamidass cautiously builds up a more detailed scenario for our history. He seeks to maximise its appeal to as many believers as possible by bringing the narrative as close as possible to a literal reading of the Bible, without increasing its exposure to scientific falsification.
This leads Swamidass to posit a surprising scenario in which Adam and Eve were created de novo a few thousand years ago, essentially as carbon copies of pre-existing humans who had evolved from apes. Their offspring then freely mingled with the existing human population, and “like a drop of water in the ocean, Adam and Eve’s genome quickly disappeared” (p. 81). Confusingly, Swamidass describes his created Adam and Eve as “monophyletic” with evolved human beings, but by this he means “of the same biological type” (p. 85), rather than the more usual definition of “sharing common ancestry”.
Given that he invokes a miracle, one might assume that Swamidass includes within it some kind of biological, mental or spiritual (“ghostly”, to use an archaism) advance unique to Adam and Eve. He explicitly eschews such novelties. This shields his scenario from the possibility of scientific falsification (though not, of course, from philosophical arguments that miracles are anti-science), but makes the proposed divine intervention rather pointless and arbitrary.
To avoid the possibility of testing, Swamidass seeks to have no objective criteria external to the Bible that define Adam and Eve as distinct from other humans. He rejects terms like “philosophical humans” to describe them and instead prefers the term “textual humans” – in other words, what the Bible means by humans is what the Bible means by humans. Thus, he allows believers to define humans in a manner that is entirely decoupled from science, and concomitantly, irrelevant to science.
There are some features of this proposal that are attractive. It seemingly diffuses an area of tension between science and faith by allowing belief in a literal Adam and Eve who lived a few thousand years ago, seemingly without any conflict with current science. There are no doubt many theological objections to Swamidass’s ideas (see, for example, here). In terms of how his ideas interact with science, I see two areas of weakness.
1) Dating uncertainty
The appeal of Swamidass’s proposal rests on our beliefs about the certainty and age of three dates: (A) the most recent date at which all humans could have passed through a bottle-neck of two, giving a pair of universal genetic ancestors; (B) the most recent date at which all humans could share a pair of genealogical ancestors; (C) the date of Adam and Eve that seems to be implied by the Bible.
Swamidass’s proposal is most compelling if we believe with a high degree of certainty that (B) and (C) are the same and much smaller than (A). Swamidass argues that this is the case. But what if (C) was in fact uncertain, as many Christians argue? Or (B) was much higher than suggested by Rhode et al (2004)? Or (A) turned out to be smaller than we had imagined due to the presence of previously unaccounted for factors shaping the molecular evolution of African populations (such as a ghost lineage, or complex past metapopulation structure, for example)? In these cases, his scenario would be less attractive to believers.
Because the possibility of a real Adam and Eve is not a topic that the majority of geneticists are interested in, the literature on (A) and (B) is extremely limited, and the subject has not been fully explored. There may well be plausible scenarios in which (B) could become too large for an easy fit with (C) or by which (A) and (B) become close enough to one another for Swamidass’s scenario to lose much of its appeal.
For example, a more recent model than Rhode et al (2004) for estimating (B) was published by Kelleher, Etheridge, Véber and Barton (2016). This gives a much older date for the common genealogical ancestors of humans. Swamidass dismisses this study as (p. 59) “less relevant” because it “unrealistically” restricts migration to only a few kilometres. However, there is clearly room for debate, and future studies that include genetic data in order better to parameterise migration in genealogical models may give estimates that force Swamidass into a scenario that works less well than his current one.
2) The ghost in the machine
Another area of potential difficulty for Swamidass’ scenario is the origin of human consciousness: the “ghost in the machine”. Swamidass’ main focus is on reconciling Biblical and scientific dates for Adam and Eve. But for many believers, this issue may be peripheral compared to the conviction that the story of Adam and Eve is related in some sense (regardless of whether it is historical or mythological) to the origin of the soul. Previous attempts to reconcile Adam and Eve with evolution have tended to avoid a physical miracle but posit a non-physical divine intervention conferring human consciousness.
It is not unreasonable for someone with a prior belief in God to think that the origin of human consciousness might involve more than just physical processes. The difficulties of explaining consciousness in terms of material processes and Darwinian evolution have been recently explored by the atheist philosopher Thomas Nagel in his book Mind and Cosmos. Christian philosopher Richard Swinburne argues something similar in his book The Evolution of the Soul. In chapter 4 of his book The God Delusion Richard Dawkins suggests that the origin of consciousness is “even more momentous, difficult and statistically improbable step than the origin of life”. It is not surprising therefore that theists would tend to see fingerprints of the divine in the origin of consciousness, and especially human consciousness. That Christian theists should associate the latter with Adam and Eve is understandable, as St Paul writes: “The first man Adam became a living soul” (1 Corinthians 15:45).
Superficially it might appear that such a spiritual intervention could easily be combined with Swamidass’s view, but as he works through his detailed scenario in the latter half of his book it becomes clear that such a combination would be highly problematic. If his genealogical Adam and Eve were the first to have human consciousness they would be objectively differentiated from evolved humans. This could open up his scenario to scientific testing and could also imply that some members of Homo sapiens were sub-human; both of these possibilities he wishes to avoid. Therefore, he concludes that, long before Adam and Eve, people had “minds and souls” and “science legitimately tells us the story of how they arose” (page 175). Believers may find this decoupling of Adam and Eve from the origin of the soul more questionable than an ancient date for an Adam and Eve who could be universal genetic ancestors of all humans with “minds and souls”.
Given the surprisingly recent date at which shared genealogical ancestors arise in populations, it was well worth exploring how this might fit with the age-old belief in Adam and Eve. Joshua Swamidass does this in a highly detailed and truly inter-disciplinary manner. He shows respect for all sides, sincerely wanting to find a way forward that can defuse an area of conflict. It is to be hoped that this book will motivate the more sophisticated modelling of the human population history. It may also make believers ask questions about their hierarchy of beliefs about Adam and Eve. Is the most important thing about them the time in which they existed, or something that made them objectively unique? Are they genetic ghosts, or ghostly ancestors? The book has less to say to the atheist or agnostic reader, except perhaps to convince them that Christian views of Adam and Eve could be irrelevant to objective reality, and to persuade them that there are reputable scientists who take both science and religion seriously. No doubt Joshua Swamidass will be on this year’s shortlist for the Templeton Prize.