The hormone - which they have chosen to name ELABELA - is only 32 amino-acids long, making it amongst the tiniest proteins made by the human body.
The team led by Dr Bruno Reversade carried out experiments to determine ELABELA's function, since its existence was hitherto unsuspected. Using zebrafish designed to specifically lack this hormone, they uncovered that ELABELA is indispensable for heart formation. Zebrafish embryos without this gene had rudimentary or no heart at all.
The team also found that ELABELA uses a receptor previously believed to be specific to APELIN, a blood-pressure controlling hormone.
This receptor called APJ or Apelin Receptor has dual functions - it first conveys signals from ELABELA and then from APELIN. Mutations in the Apelin Receptor also prevent the heart from forming.
Zebrafish bereft of the Apelin Receptor are referred to as the Grinch, in reference to the cold and heartless cartoon character created by Dr. Seuss in 1957.
ELABELA has also been found to be expressed in human embryonic stem cells, indicating that it might have other functions beyond its role in cardiovascular development.
As some people might have a harmful copy of the ELABELA gene in their genetic make-up, sequencing and screening for this particular gene in the general population might also help to detect predisposition to heart anomalies before the disease progresses.
Their results have been published online in the journal Developmental Cell.