What step in the evolution of life is the most complex and occurred last?

Answer 1

Evolution is the change in allele frequency over time in a population of organisms.

So, evolutionarily speaking your question does not make sense. Evolution is not stepwise, not directed towards a goal and complexity is, arguably, in the eye of the beholder, though organisms tend to become more complex over time there is no " last " in evolution.

That said, high intelligence in advanced mammals may be said to be complex and last here.

Although, alternatively, it could also be said that the evolution of intelligence is, although perhaps not a necessary consequence, still an expected consequence of the evolution of the central nervous system, itself an expected consequence of the evolution of nervous systems, in turn an expected consequence of the evolution of multicellular organisms.

It has been argued by some that the emergence of multicellularity is the most complex step in the evolution of life on Earth to date, and could be argued at least as improbable as the evolution of high intelligence [Ward & Brownlee 2000, Grossberg & Strathmann 2007]. However, biological and computational experiments have, since this was first argued, shown how multicellularity might evolve rather quickly, given the proper circumstances [Travisano & Ratcliff 2012].

Others have argued that it is the evolution of not intelligence but self-awareness that is so complex that it might not have been expected. They reason that it is perfectly possible for intelligence to emerge and act favourably upon its own preservation and promotion without a sense of self [Metzinger 2003, Wegner 2002]. They argue that self-aware sentience is costly in terms of computing time and the resources required to uphold it [Matsumoto & Tanaka 2004]. As the phenomenon of sentience is still poorly understood, it is difficult to determine how complex or improbable such a development is, exactly.

How complex something is generally is a matter for discussion, I'd say. For instance, while we might argue that the Solar System is a complex whole of many moving bodies, it is also true that this whole follows naturally from the simple rules describing the motions and interactions of these bodies. How complex or simple does that make the Solar System?

Answer 2

The evolution of eukaryotic cells from prokaryotic cells is considered the most complex step and occurred later in the history of life on Earth. This transition involved the development of membrane-bound organelles within cells, such as the nucleus and mitochondria, leading to increased complexity and diversity of life forms.