Evidence is piling in that within extreme forms of bacteria called Archaea there is a group called Asgard with DNA fingerprints suggesting ancestry to cells like our own. It appears that all eukaryotic cells were created by a single endosymbiotic event in which an archaeal bacterium ingested a smaller non-archaeal bacterium which was able to use the oxygen we must breathe to survive. Obviously, a secondary endosymbiotic event was required to imcorporate a photosynthetic bacterium into this cell in order to create algae from which higher plants evolved.
At this point, all speculation screeches pretty much to a halt because mainstream science is at a loss for how such processes as cellular differentiation required for fetal development of animals like us came to be. They are well aware that other endosymbiotic events have occurred beyond just two, but they seem unable or unwilling to connect the evolutionary dots any further.
Let me provide some additional explanations here by first asking another fundamental question: Why are animal cells the only kind of eukaryotic cell that produces such a wide range of complex organisms with hundreds of different kinds of cells? In humans we have 200 different kinds of cells. In higher plants, there are basically three kinds of cells and in higher fungi like mushrooms there are basically just hyphae filaments and fruiting bodies that generate spores. The answer seems to be what lies next to the outside of the cell membranes of these three kinds of eukaryotic cells. Both plants and fungi have cell walls which may limit the number and kinds of endosymbionts they contain. No such walls exists in animal cells. They are free to take up numerous symbionts above and beyond the two they share with plants and fungi.
Imagine for a moment, two cells merging together and sharing their DNA within a single nucleus. This hybrid cell may generate additional clones comprised of one or the other original cell types based upon local environmental changes. One cell functions better in one kind of environment and vice versa. Over time, DNA redundancy may be reduced so only unique genes of each cell type are retained plus what are known as “housekeeping” genes required for the survival of both cell types. This kind of cell merging can be repeated time and again, each time losing DNA redundancy to limit cell size. These cells begin to form a colony of different cell types, each one contributing to the size, shape, motility and other factors that provide the optimum body plan for survival. This may be the beginning of animal embryogenesis.
Another fundamental question about animal embryogenesis is the seeming paradox of cellular differentiation as irreversible versus reversible. For example, why can salamanders regrow a limb while humans cannot? This requires an intimate understanding of how DNA is arranged within the chromosomes. Obviously, all of the DNA for growing a limb must still exist within the adult salamander and is readily available for regenerating a limb upon amputation. So why can the same thing not be said of humans? Scientists have been trying for years to force differentiated cells back to a more primitive state or induce primitive embryonic or stem cells into differentiated cells required to regenerate necessary tissues. These issues have been discussed in detail in older posts within this blog. I will reiterate as follows: It appears that there are at least two basic controls involved with cellular differentiation: 1) chromatin condensation which restricts access to some of the genome with no DNA loss, and 2) irreversible loss of genetic material; hence the paradox of irreversible versus reversible cellular differentiation. It may be possible to shut down one active genetic compartment through chromatin condensation while opening up another one. In this case, the newly opened compartment may lose DNA, just like the older one (not the same DNA, of course) in order to fullfill the requirements for differentiation into a new kind of cell type. The crux of my blog posts is to explain how irreversible DNA differentiation might be accomplished based upon a series of endosymbiotic acquisitions.