in bacteria that is! Why can’t (or won’t) bacteria which are single celled organisms (parkaryotes) join together to form complex molecules or multicelled organisms (eukaryotes)?
The reason is lack of specialized battery for supplying the huge energy requirements of complex organisms.
It’s well known among scientific community that about four billion years ago, parkaryotes combined with mitochondria which was on it’s own another parkaryote. Once this combination occurred, a symbiotic relationship got established. Mitochondria became specialized in producing the energy needed for protein synthesis and cell division. The multicellular organism requires enormous amounts of energy (ATP and ADP) compared to the parkaryotic cousin because of the huge magnitude of genes.
This work is published in Nature on a paper titled Hypothesis by Nick Lane & William Martin.
Note this is a Hypothesis and the authors encourage other scientists to try and disprove the hypothesis. The existence of a primitive eukaryote without mitochondria would disprove it (they note that all eukaryotes had mitochondria at some point, although some have lost them). Another problem for their model would be giant prokaryotes with either membrane associated plasmids devoted to generating bioenergy or with high respiratory rates but without lots of copies of their genome. A prokaryote with a haploid genome the size of a eukaryote’s would disprove it as well.
Until then this Hypothesis will hold.