This is an excellent and very advanced question… Yes and no. We do tend to see an increase in complexity…but it doesn’t have to work like that. Think about our vestigial appendix that someone mentioned earlier – we don’t need this anymore and so it has gradually declined. The bottom line is that evolution will change a species to fit within its environment – this could be a very simple solution to a problem, or a very complex one!
What an intelligent question! We are always tempted to believe this since we imagine organisms early in our evolutionary history to be incredibly simple. However, the more we study these organisms the more we realise they are just as complex as vertebrates! For example- we used to think jellyfish were incredibly simple organisms compared to vertebrates yet when we studied their genomes we realised that they had nearly as many genes as much ‘higher’ organisms. We used to refer to organisms like sponges and jellyfsh as ‘basal’ organisms and vertebrates as ‘higher’ organisms because we though evolution was all about building on simple organisms and making them more complex. We now understand that isn’t the case. Many of the genes responsible for creating the overall architecture of our own bodies are present in most other organisms be they worms, birds, flies, sponges or snakes.
The fossil record can be quite misleading too since we never get a complete view of past ecosystems when we look at fossils, there are very many species missing from our fossil record because the simply didn’t get fossilised. This persuaded many scientists in the past to consider that life used to be much simpler on this planet. However, complexity is one thing that has remained constant throughout the evolutionary history of Life, but we’re only just starting to appreciate this through studying animals’ genomes.