The phrase "first vertebrates" refers to the very first animals that had a backbone or spine. These were the earliest creatures with a skeletal structure, and they played a crucial role in the evolution of life on Earth. Full definition
Sex hormones, including testosterone, evolved 500 million years ago — before the first vertebrate animals — from the «mother» of all steroid hormones, estrogen. (discovermagazine.com)
The first vertebrates to forsake the seas probably didn't walk or even crawl, however. (discovermagazine.com)
«For the first time we can definitely show that RNA viruses are many millions of years old, and have been in existence since the first vertebrates existed. (sciencedaily.com)