The assumption that evolutionists make before interpreting their observations is that all creatures share a common descent. This means that all creatures can trace their ancestry back to the same simple original cells. The evolutionists therefore assume that evidence will be found eventually for simple creatures in Precambrian rocks that developed and diverged over time into all the kinds of body plan we see today.
The term LUCA refers to a mythical last universal common ancestor from which all organisms now living have descended. Scientists compare the metabolism of all extant organisms to try to figure out what features are shared by all, and they then hypothesize that these were present in the cells that were ancestral to all kinds of modern life. On this basis they conclude that the last universal common ancestor was by no means a simple form of life.
For example, a preprint manuscript released in 2020, declared in the abstract: “Our results depict LUCA as likely to be a far more complex cell than has previously been proposed, challenging the evolutionary model of increased complexity through time in prokaryotes. Given current estimates for emergence of LUCA we suggest that early life very rapidly evolved considerable cellular complexity.”
You can see that these studies are based on a lot of evolutionary assumptions and there is no way to check whether the assumptions or the conclusions might be reasonable. The conclusion that complex cells appeared quickly after the origin of life, does not fit evolutionary expectations. As far as we are concerned, every type of life was created during creation week and these unprovable speculations of a common ancestor are futile and unhelpful.
Evolutionary trees are plotted on the assumption that once life appeared, populations which had become separated, increasingly displayed different characteristics. Over time, as a result of chance mutations and some creatures doing better in some environments than others, populations which originally had belonged to one group, then continued to show greater and greater diversity. This process brought about major differences in body plan as the millions of years passed by. Discussions on Cambrian explosion, body plan and evo-devo demonstrate that these are based on evolutionary assumptions and do not fit our observations from the real world.