The first major evolutionary change is that all animals on the tree are Deuterostomes. This classification points towards the first opening on an animal as it develops being the anus (as opposed to the mouth).
The second major evolutionary change between the Sea Stars and the Sea Lancelets is the Notochord present at some point in development.
The notochord is a skeletal rod that supports the body during development, and sometimes stays around in adults. It comes from the mesoderm, provides skeletal support, and in some vertebrates the notochord ends up being disks between vertebrae.
Third major evolutionary change, between lancelets and lampreys, is the development of a head and vertebral column.
The vertebral column is made by vertebrae and intervertebral disks. Also known as a spine. It is necessary for supportof the organism’s structure and flexibility.
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