Re: The first organ that gets formed is the anus. And some modern day ancestors stay that way into adult form (or at least externally adult in appearance). On

Re: The first organ that gets formed is the anus. And some modern day ancestors stay that way into adult form (or at least externally adult in appearance). On a serious note, it looks roughly slug-like. Most guesses of the earliest animal were more "wormy", that is, narrow. It's also odd that Spriggina and Dickinsonia left no ancestors (that we know of). The "earliest mover advantage" does not always play out. They are akin to the Apple Newton of smart-phones/PDA's. (Plus, having naughty-part sounding names probably didn't help). I suspect Spriggina and Dickinsonia were single-celled organisms. (Even today thumb-sized semi-mobile cells exist.) That gave them an early advantage when oxygen first became plentiful because they didn't have to coordinate millions of cells, but rather merely grow bigger. However, multi-cellular eventually caught up and had scalability/complexity advantages over one-big-cell. The digestive track may be such a result. Spriggina and Dickinsonia probably more or less just sat on stuff and slowly digested it in place. The new system allowed Jabba the Mini-Hutt to eat on the go, useful as predation increased or for food sources too small to bother sitting on. We see similar patterns in social animals and symbiosis in general, such as flowering plants. They are slower to appear on the evolutionary stage, but eventually out-compete creatures that rely on simpler behavior. Neanderthals had bigger brains, but were probably less social. Trade and sharing ideas gave us more tools and food sources. After the "Dino Asteroid", birds had the early predator advantage and grew quite large and fierce. However, the social pack hunting of mammals eventually gave them the edge.