Animals are divided into two main groups: Animals with a backbone are called vertebrates. Animals without a backbone are called invertebrates. Vertebrates and invertebrates can be sorted into ...

Among them were the chordates, to which vertebrates (animals with backbones) such as humans belong. What sparked this biological bonanza isn't clear. It may be that oxygen in the atmosphere ...

It could be seen in the segmented bodies of worms, in the vertebrae of animals with backbones, in the head-thorax-abdomen construction of insects. In all of these, modular units like Lego TM ...