Sabbe dhamma anatta. This is probably the most radical statement in the entire existence of the world’s religious and philosophical thought. At the same time, it was made by the Buddha in a context that was - and still is - highly, if not exclusively empirical and practical, just as the Dhamma taken as a whole. This means that application of the anatta teachings to theoretical contexts does not carry as much (if any) value, but I think that a theoretical discussion held in the spirit of the Right Speech and Right View can still help us progress on the Path.
So, we have no self. Moreover, we consist of five groups of phenomena called khandha. Of these five, at least half a khadha, namely rupa, is demonstrably composite and consisting of separate organs and crucially, cell. Biologists researching the human perception can also confirm that it is a multi-level process consisting of multiple elements and sub-processes. My intuition is that all other khadhas will also be found to exhibit composite nature. Combined with anatta, the concept of a living being starts reminding an avalanche or rain, a natural processes consisting of numerous elements, intricately connected to each other. Still, the doctrine of kamma is interpreted in a very linear way, as if there was something reminding an existential core of a living being. One being - one set of kamma. You do an unwholesome thing, you to go Hell, you do a wholesome thing, you reap good kammic fruit.
Problems arise if we start to apply this teaching to living beings that are quite unsimilar to us. Take a rainworm: you can cut it in two parts, each of which, as some reasearchers suggest, may retain ‘memories’ of their life prior to the separation. Which of these two halves retains the kamma of the ur-worm (provided worms have kamma)? I think the only meaningful answer is ‘both’. Indeed, if a living being is an impersonal natural process, ontologically not vastly different from rain, why should it be impossible for it to split in two? There was only one worm with only one set of kammic structures, then boom! there are two worms with two initially identical but exceedingly divergent sets of kamma and rebirths. While there is no evidence in support of this theory in the Suttas, there is - as far as I know - nothing contradicting it. Moreover, it is hard to imagine that it was one of the Buddha’s immediate concerns to enlighten us on the nature of kammic transition in rainworms.
This scheme of ‘splitting beings’ may also be applicable to cells. Most of the cells in our organism behave as separate living being with a varying degree of specialization, something quite similar to how a developed (post-)industrialized society is structured today. In fact, our bodies are nothing but a huge colony of huge colonies of monocelular organisms, each having its own ‘body’ and mechanisms interacting with their environment in specific ways, i.e. proto-ayatana. Some of these cell, as leicocytes, are so highly autonomous that they can hardly be perceived as parts of our bodies. If so, why can’t they have their own kamma? Why can’t we as living beings with our kamma consist of other living beings with their own kamma? And finally, what happens to the kamma of a cell during the mitosis?
The latter question is especially important since every single one of us has developed from a single fertilized cell. Whether this ferticilized egg cell right after the moment of conception is already a human being is a debatable question - its being debatable is precisely what emphasizes the conventional nature of the word ‘human’ (cf. to the discussion of the term ‘Tathagata’ in SN 22.85 and the simile of a chariot in Milinda Panha: Where is ‘human’ in a human being? When does a human being starts being human? When does it cease being human?). Of course, one possible interpretation is that it is already human in the sense that it acquires the khamma of the previous birth and serves as the kammic foundation of an entire human organism. If we assume that all living being (and possibly all matter) possesses some extremely, unimaginably primitive sentience, the ‘consciousness’ of this cell could serve as the basis of the further biological development and correspond to the vinnana nidana in the interdependent relationship of vinnana and namarupa as described in DN 15. In the process of mitosis there would emerge a ‘cellular consciousness’ of each cell in the embryo as well as the ‘overall consciousness’ of the whole organism. More importantly, the same is true for the kamma: there would be not only ‘overall kamma’ of an organism but also ‘cellular kamma’. An alternative explanation, found among other sources in works of Ven. Brahmavamso, is that consciousness arises in an embryo at a later stage, while the earlier stages cannot be referred to as ‘human’. The venerable Ajahn did not spell it out for obvious reasons, but this interpretation - just as the one I provided above - can have important consequences for the ethics of abortion, even though I do not find it in any way more convincing than its alternative.
Besides, we should ask ourselves: if living being can get split, can they merge? The most obvious example would again be fertilization of an egg cell by a spermatozoid. Each of these two cells are highly specialized monocelular organisms - may I say living beings? I wouldn’t say their merging means they both die, a more appropriate way to describe it would be two say that they merge as two rain clouds can merge with each other into a single entity. Wouldn’t it mean that their kamma merges as well? This could also explain why the research of reincarnation evidence of Dr. Stephenson et al. reports about people being ‘reborn’ while their previous incarnation is still alive. I see no reason why living beings and importantly their kamma could not ‘split’ or ‘grow out’ of the maternal entity. Surely, the Buddha did not say anything about this in the Suttas, but are these details really so important for the Dhamma practice, especially if all his disciples were certain that the rebirth exists? He also said nothing about the theory of relativity, quantum mechanics, organic chemistry, and successful strategies of coaching a baseball team, which doesn’t mean that they are not relevant in their specific contexts.
Finally, everything I wrote above may give us a tiny bit more intellectual insight into what happened under the Bodhi tree during the Night of Enlightenment. I think there are relatively few people who would subscribe to the opinion that it was not only the Buddha but also his colon cells that became enlightened that night. So, an enlightened being literally consisted of myrads of unenlightened ones. Moreover, according to the Yamaka Sutta, it would be an impossible task to pin down what exactly is enlightened about the Buddha, since you can’t find Tathagata in any of teh five khadhas. What did the Enlightenment consist in, then? Dare we say that the Enlightenment happened on the discursive level, where we all exist as ‘human beings’, with the change of the descriptional perspective when the Lord Buddha saw Himself and the world as they really are: “Sabbe sankhara anicca, sabbe sankhara dukkha, sabbe dhamma anatta”?