I’ve been thinking about a wild theory regarding the incredible biological complexity we see in mammals today.

What if our bodies (apart from the brain) are actually the result of an ancient aggregation of once-separate "organisms" that evolved to live symbiotically?

Over millions of years, their DNA might have fused and co-evolved into a single, unified genome. What began as cooperation between distinct life forms could have gradually become inseparable, giving rise to the intricate multicellular systems we now take for granted.

It's called Symbiogenesis [0] and it's not at all a wild theory. But it's limited to cell components, not multiples organs fusing to create something as complex as a mammal.

[0] https://en.wikipedia.org/wiki/Symbiogenesis

This isn't a wild theory or a novel one. It's well-established that endogenous retroviruses alter DNA and are inherited. In addition to the primary genome being modified this way, all mitochondria are symbiotic organisms inside plant and animal cells, with their own DNA, and are vital to life. Same thing for chloroplasts in plants. And then there are gut bacteria, which are vital to life, symbiotic, and directly influence evolution and the genome.

https://en.wikipedia.org/wiki/Intragenomic_conflict

You should look into the origin of mitochondria.

I believe that we’re living in that situation now. I don’t think life can be divided into smaller organisms. That there is just one complex life that we failed to see based on our past prejudice.