Answer the following question given this paragraph: 

The first vertebrates evolved about 550 million years ago. The evolution of amphibians, reptiles, mammals, and birds occurred over the next 400 million years. The first vertebrates were ectotherms; endothermy evolved later. Modern fish, amphibians, and reptiles are ectotherms. Modern birds and mammals are endotherms.


Q: Vertebrates differ from invertebrates because they lack this?

 Choices:

- legs

- head

- neck

- backbone or a spinal column

A:
backbone or a spinal column
