We are all decimal men (and women)
This article from Bytesizebio points out that our bodies are only one-tenth human: in our bodies, human cells are outnumbered by the cells of harmless or symbiotic microorganisms, about 10 to 1. Wikipedia concurs. 
What it means to be human, when 90% of our body isn’t? (To be clear, human cells take up a larger volume than our passenger microbes, as the latter are smaller.) And that other nine-tenths can literally shape our lives; as the article explains, the bugs in your gut can make you fat!
Most interestingly, Part 2 of the article divulges that while identical twins share the same human DNA (their physical differences apparently being due to variations in gene expression) their gut flora populations are as unrelated as those of randomly-picked strangers. In other words, while identical twins may share essentially the same human cells… they’re as different as any pair of people, when it comes to the other 90% of their body!
The idea that most of the cells in our body aren’t actually human flashed me back to the Buddhist concept anatman, which posits that there is no intrinsic, fixed, monolithic self. To use the Western term, there is no intrinsic “soul”.
As an aside, Buddhist doctrines on reincarnation make for fun reading, given that there should be nothing to reincarnate! The most intellectually satisfying argument I’ve read argues for reincarnation-without-transmutation. The analogy would be of a lit match lighting a second match, then going out. The fire lives on — it has been “reincarnated” in the second match — even though the flame from the first match has gone out. One’s personhood disappears, but its effects can ripple onwards, like the ripples of water in a pond into which has a pebble has been thrown. This is very different from most notions of reincarnation, which tend to involve one’s personality making a return trip to life’s merry-go-round.
In the Buddhist perspective, one’s personhood is a transitory product of the interaction of various factors — genetic, environmental, cultural, and so forth. (This is the idea of dependent arising.) As the factors acting upon it change, so does the personhood; there’s nothing intrinsic that remains the same. As people grow from youth to maturity to senescence, their character may change with their circumstances. A sorrowful example would be the changes in personhood undergone by patients as dementia envelops them. The previous personality ceases to be. Presumably, reincarnation / resurrection enthusiasts believe the personhood of such individuals would return in pre-disease form.
In an analogous way, scientific inquiry has shown us that our bodies are not intrinsically human; indeed, without symbiotic microflora, our digestive system wouldn’t work properly. Rather, our bodies are the product of the interaction of our outnumbered (!) human cells with hundreds of microbe species, in a reasonably symbiotic relationship. And if the ratios or populations of microbes within us change, our bodies might noticeably change, too.
Extrapolating this analogy outwards one layer, we commonly think of our communities and countries from a strictly human perspective: population, demographics, GDP, unemployment, and so forth. But this externalizes factors which deeply shape our societies — the climate, terrain, and the various biological subsystems which support us. And just as ill-health of our gut flora eventually reflects in our own vigour, historical examples exist of societies collapsing as the biological systems supporting them, fail.
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Late-breaking news from the BBC: British scientists are investigating whether various human ailments — asthma and auto-immune disorders particularly — could be cured by giving people parasitic worms. Which would imply that they aren’t parasitic. Rather, they’ve been (an unnoticed) part of our bodies for so long, that we can’t live properly without them. A sort of biological co-dependency, I suppose. For me, this was the most fascinating snippet of the article:
“Bacteria were introduced to a group of amoebae. The amoebae did not like the bacteria and tried to kill them - but could not.
And five years later neither organism could live without the other.
The amoebae had deleted certain genes in their own immune systems and the bacteria had done the same so they could coexist peacefully.
As a result, the amoebae no longer had a complete genome unless the bacteria were present.“