There are two ways to view what it means to be a human, biologically speaking. Either way determines how many cells we view as being in the human body.
The first way, which says that the human body is made up of cells which are genetically human, says there is something like 37 trillion cells for a 75 kg person.
The second way, which says the human body is a meta-organism comprised of a complex community of organism working together in the system we call the body, says there is probably twice as many cells, though the second half of those cells accounts for around 0.3% of the total weight of the body.1
These two competing ideas have, at there core, a philosophical question. Does the human body have symbiotic relationship with other beings or are human cells, meaning do we say “we have a biome”, or are human cells and human systems “merely parts of a united meta-organism whose components evolve in concert with each other”?2
It has been speculated that endosymbiosis played a part in the production of mitochondria, chloroplasts, and chromatophora. The most recent discovery of an organelle called nitroplasts is one that fixates nitrogen.3 Most plants have a biome of bacteria near their roots which free up nitrogen in the soil for use by the plant. This symbiotic relationship has benefits for the plants and the bacteria and both need each other for survival. This newest organelle fixates nitrogen but, instead of being outside of the cell in a symbiotic relationship, it instead inside the organism as an organelle. It is thought that this organelle was formed via an endosymbiotic relationship, as the endosymbiotic theory proposes.
The important thing to note here is that the line between organism and symbiote is becoming blurred. In the plant example, why is it that the organelle is a part of the organism whereas the root biome is not a part of the organism? The theory of holobionts makes the bold statement that a plant is a meta-organism and that the root biome is, in fact, a part of what it is to be a plant.
See The Economist. “The Idea of ‘Holobionts’ Represents a Paradigm Shift in Biology.” Accessed April 24, 2024. https://www.economist.com/science-and-technology/2023/06/14/the-idea-of-holobionts-represents-a-paradigm-shift-in-biology?utm_medium=cpc.adword.pd&utm_source=google&ppccampaignID=17210591673&ppcadID=&utm_campaign=a.22brand_pmax&utm_content=conversion.direct-response.anonymous&gad_source=1&gclid=CjwKCAjw26KxBhBDEiwAu6KXt3IdbXKxUWIpCaeFoHHBx98Vat42zz3z49NP-cuXHJjypQy5PVzX8xoC6OQQAvD_BwE&gclsrc=aw.ds.
Ibid.
See Incredible Discovery of an Entirely New Organelle That Fixes Nitrogen, 2024.
.