Bacteria can change quickly, but do not evolve into a new species, that is not also a bacteria. The main problem with the bacteria mode of evolution, for higher forms of life, is bacteria make too many changes in terms of a family of siblings. Multicellular creatures would self destruct, if all their cells, down to each organ mutated, throughout their entire development, as fast as bacteria do. Picture every brain cell of the fetus, being slightly different at the genetic level, before they even add branches. The heart and liver would be a mishmash .
What had to happen for the advancement from single to multicellular, was the mutation rates had to be better controlled each cell cycle. Today we have proofreader enzymes to correct these defects. The proofreaders would not benefit bacteria, unless there was a cause and effect mechanism for their need to change.
With bacteria, since each is it own entity, they can mutate quickly, side-by-side, so at least some extended family member can survive or become optimized to any situation. From there a new colony can grow. But higher animals can't just partition all their organs into a dozens of mutant sub-organs parts until one aspect works better and then take over the organ, Evolution itself had to evolve; step up, with better quality control.
With multicellular differentiation, all the different cells use the exact same DNA; like a single mother bacteria. But there is much higher quality control over future of the DNA, with each differentiated cell, using just part of the same DNA. This is done by dialing in the configurational potential. The DNA of the daughter cells starts as condensed chromosomes. Based on how this is unpacked, will define which genes will be used, and therefore which cellular differentiation.
The DNA packing protein are oily or contain long organic side groups; histones (lysine and arginine). The zigzag lines are the oily.
Protein rich in these amino acids create surface tension in water. This surface tension can be lowered by being shielded by the DNA. The DNA will wrap around and help shield the water, from the histone's water and oil effect. As we unpack the DNA, this exposes these protein, again and will increase the surface tension of water, unless we can dispose of the packing proteins; send them to recycle. If we recycle these protein, the unpacked DNA is now at even lower potential, by losing that oily core. There is a difference in water potential between packed and unpacked DNA. As we unpack the DNA and dispose of the packing protein, more of the DNA configuration favors the water side and less the oil side.
One aspect of the DNA always stays packed, except when the DNA is duplicated; centromere. The centromere region is like the icy oily pole of the DNA's configurational gradient; frozen/packed solid. The most unpacked aspects of the DNA are the warmer pole. Based on the ratio, we can tune in any differentiated cell types. Often the last packing protein exposed are not recycled, but remain nearby, to repack the DNA, and adjust the configurational potential to its differentiated sweet spot.
In terms of maintaining the cellular differentiation control, in the human body, so all the organs stay steady, is the gradient created between the blood supply and nervous system found near most of the cells. The blood is the oil side and the nerves represent the water side. The brain also has a blood-brain barrier, which separates the brain, to some extend from the oily side.