By looking at cartilage formation in an organism closely related to vertebrates, scientists learn how vertebrates evolved their unique head structure.
The animals that most people think of (humans, cats, dogs, fish, etc) are all vertebrates. Vertebrates are defined by their spinal cord which sends signals from the brain to the rest of the body. However, an additional feature of vertebrates is their noticeable head which is protected by a skeleton. How this protected head evolved is still an open question in evolutionary biology.
It is currently thought that the first vertebrates evolve to protected their head with cartilage. Then, over evolutionary time cartilage was replaced with more durable bone. This transition is mimicked in developing vertebrates today: babies developing in the womb protect their head with cartilage which is replaced with a skull made of bone later in development.
But how did the transition to a protected head evolve? One way to address this question is to look at close relatives to vertebrates and make assumptions on how they could have transitioned to have vertebrate-like heads.
By examining the lancelet, an invertebrate that is closely related to vertebrates, scientists have traditionally assumed that vertebrates evolved to protect their skull with cartilage using specialized cells called neural crest cells. Neural crest cells, which make protective head cartilage, are not present in lancelets. Therefore, scientists reasoned that these cells were utilized in the evolution of a protective head in vertebrates.
A recent study from biologists at the University of Colorado questions this previous model of evolution. Although lancelets clearly lack neural crest cells, scientists now realize they make cartilage during their development in a manner that resembles vertebrate development. Examining the embryonic development of lancelets, it turns out that their oral skeleton develops through a cartilage intermediate that reflects vertebrate head skeleton development.
Since relatives of vertebrates can produce cartilage during development without using neural crest cells, it seems that vertebrates did not rely on the creation of specific cells (neural crest cells). Instead, it seems more likely that neural crest cells simply co-opted the cartilage forming pathway and utilized it to make a protective barrier around the head.
While being a fundamental shift in how we understand the evolution of vertebrate head skeleton, this also represents a common theme in evolution. Rather than create a whole new pathway to make a head, vertebrates just took a pre-existing pathway and modified it. To make their protective skull, they utilized an ancestral strategy of making cartilage and started making cartilage in their head. Why reinvent the wheel if you can just move the wheel to a new car?