It is still accepted on geologic timescales, as you mentioned, and exactly as the sentence stated, "as you go back in time, life forms get simpler". We started

It is still accepted on geologic timescales, as you mentioned, and exactly as the sentence stated, "as you go back in time, life forms get simpler". We started with lipids and other molecules that then appear to have formed membranes. Amino acids collected in those membranes, formed longer self-replicating molecules, and prokaryotic organisms. Eventually you wound up with incredibly simple, chemical ingesting single cell life, and not long after, eukaryotic organisms with separate nuclei. This then gave way to multi-cellular colonies, which then lent itself to cellular specialization in multi-cellular organisms. Then you start to see highly specialized organisms like plants and animals. Complexity doesn't mean body parts, it means specialization within an organism and within a species in the beginning. We see that clearly. Specialization into new niches and adaptations to changing ones continues even if complexity does not, presently. However, we still might see something evolve into a more complex, 5 dimensional, dark matter eating, electro-plasma brained creature.