Enn's case seems to overreach. But the issue itself is pressing. The reason is that the evangelical gospel as it's told in the Scripture requires a historical Adam to make sense of it's account of redemption, as many theologians have argued. The problem is that there seems to be no reason to think that there could be a single first ancestor to the human race.
One of the surprising features of the fossil record is not merely the paucity of apparent transitional forms, contrary to what Darwinian evolution expects, but also the sudden "explosions" of the appearance of new species, starting with the Pre-Cambrian Explosion. Millions of years pass without any great sign of a species. Then suddenly not only does one appear but a lot do in a very short period of time relative to the scale of evolutionary history.
I am a philosopher or at least I play one at school, and I wonder if philosophy may have any light to shed on this. This suggestion comes from several sources, from Plantinga to Pope John Paul II, but the suggestion is mine.
Assume for the sake of argument that the theory that several humans (say 250,000) appeared more or less at the same time all over the accessible continents. This is certainly a scientifically testable hypothesis. It may be what science can speak to.
But we also know more keenly is that science confronts one of it's epistemic limits in trying to relate the mind to the brain. This was a key point in the methods of science and phenomenology in the Pope's statement on evolution. The phenomenological and intentional character of thought is impossible to relate to the dynamic and extensional categories of science and thus it is difficult to relate the mind to the brain. Some vehemently dispute the idea that mind cannot either be reduced to or explained away by the physical features of the brain. For now, I only point the reader to the intractability of the debate over this even among philosophers with no theistic ax to grind.
This means that while science can tell us that there are likely to be 250K original samples of the Hunan species, it may not in principle tell us about whether or not they were "minded". No doubt that sounds implausible since just as we would assume that as a member of a species, a feature of a zebra will also be true of many other zebras, then a feature of a human will likely be true of other humans. And since the other humans of our acquaintance have minds so did all the original ones.
But having a mind is not a typical feature. One of the features of the mystery of mind and brain is the emergence of an agent mind from non-minded animals in evolutionary history. Again the categories seem to make difficult any gradualist account of the emergence of mind. Suppose that God exists and set the initial conditions of the universe to arrange a point in evolution to produce a myriad of human species to make the result more likely that one of them would become minded.
So it's not epistemically impossible that there is only one Adam given the limits to the categories of science. It's important to see that this is not merely a lack of data but based on a principle limitation to our inquiries.
In conclusion, science is not able to rule out the possibility of a first minded human who stands apart from an unawakened human herd, and this is because of the natural limits of empirical science.