Animals never just started out as being classified among 9 different phyla. It started off with a single group known as the Ancestral colonial protists or Ancestral stock.
From this stock, multicellular organisms developed. These multicellular organisms then later branched off into two separate directions. One branch turned into organisms which were asymmetrical, with no true tissue layers and no gut although it has two cell layers – the inner layers consists of collar cells and between the cell layers a jelly-like layer, containing tiny spikes, is found – (Phylum Porifera), whilst the other started to develop true tissues.
Multicellular tissue organism, second branch from the ancestral stock, differentiated further to become two layered and three layered organisms.
Those multicellular organisms which had two layered tissues developed radial symmetry along with a gut with one opening – meaning the same opening is used for food intake and waste disposal - and became known and Cnidarians compared to those multicellular three tissue layered organisms which developed bilateral symmetry.
Bilaterally symmetric organisms are but a large group for many different phyla today. There are 7 different phyla which all have a bilateral symmetry as well as three layered tissues made up of multiple cells but to get to them, we first need to undergo a few more transformations.
The first Split within this group leads to 2 smaller, although still large, groups. Split 1 will end up forming 5 different phyla whilst Split 2 will end up forming only 2.
Spilt 2’s 2 subdivisions are the Echinodermata phylum and the Chordata phylum. Chordates, the phylum which humans belong to, are characterized by their rod of stiffening tissue to support the body which has a nerve cord running alongside the rod – ending up by the brain. They would be closely related to Annelids and Arthropods but they have a dorsal nerve cord....