The step from single to multicellular life forms is a metasystem transition, and those don’t roll back, and are amplified by branching growth at the penultimate level1.
Or, more concretely: A strain of cells learns to communicate with each other, to coordinate, giving all a fitness advantage in other words they create a control system to regulate the lot of them and apes together stronger than apes apart. The emergence of that (usually distributed) control system is a metasystem transition. Because that kind of cooperation has advantage over not cooperating like that, evolution never goes into the other direction (in that sense it has a direction, similar to how time doesn’t really exist in physical microstates, only in their relationship to macrostates: It’s not like genes can’t drift in the other direction, it’s that if they do they get culled at a much higher rate).
And because our critters now have an advantage, they have more resources to develop, to multiply both in absolute number, as well as to specialise into different functions. That’s the branching growth at the penultimate (that is, below the control system) level, and it makes them even more fit. Branching growth is one of those cybernetic laws that happen again and again and again and again and you’d think “surely this can’t always be the case” and yes you’ll find exceptions but by and large, yes, once there’s a metasystem transition you get that effect, again, because it’s very regularly beneficial to the whole.
If that got you consider the evolutionary step from soup of chemicals over the first feedback systems made out of simple molecules creating environments benefitting their own replication to actual cells. Which is explainable by chance alone, but once you take metasystem transitions into account suddenly it doesn’t take an eternity, any more, only aeons.
1 I should, possibly, at this point warn about Principia Cybernetica just as people warn about tvtropes. It’s a rabbit hole.
Chances are that any new large commercial platform will enshittify, sooner or later prompting another exodus, and each exodus will at least have some people choosing a community platform.