Amy Maxmen in Nature:
The head is stately, calm, and wise,
And bears a princely part;
And down below in secret lies
The warm, impulsive heart.
— John Godfrey Saxe, 1898
For centuries, writers have mused on the heart as the core of humanity’s passion, its morals, its valour. The head, by contrast, was the seat of cold, hard rationality. In 1898, US poet John Godfrey Saxe wrote of such differences, but concluded his verses arguing that the heart and head are interdependent. “Each is best when both unite,” he wrote. “What were the heat without the light?” At that time, however, Saxe could not have known that the head and the heart share a deep biological connection.
In the past 15 years, scientists have uncovered a developmental link between the two. In 2010, for example, researchers revealed that the same small pool of cells that divides and differentiates to form the heart in mouse embryos also gives way to muscles in their throats and lower heads1. Key components of the two are cut from the same cloth.
Even more surprising is that the embryonic head–heart connection pre-dates the evolutionary origin of vertebrates, and perhaps even of the head itself. Researchers stumbled on the link while studying sea squirts, blobby, sedentary marine creatures, found affixed to the sea floor, that have two openings — one for sucking water in and the other for squirting it out — hence the name.