Among the many types of hormone, one particular class — the water-insoluble or hydrophobic hormones, which include mammalian steroids and many plant-growth regulators — is particularly interesting because the chemical structures of its members are often conserved between plants and animals. This similarity is surprising because it is generally thought that the plant and animal kingdoms separated from each other in evolutionary terms long before either group became multicellular. As a consequence, multicellular growth and development in these two kingdoms are very different. So why are similar keys (chemical structures) used to unlock fundamentally different developmental locks?

The conservation of biochemical molecules between say, mice and trees, could mean that common biosynthetic pathways existed before the plant and animal kingdoms separated, and that evolution has simply used these ancient pathways to create similar-looking signalling chemicals. This seems to be true for steroid-like hormone synthesis, as plants and animals have many biosynthetic steps in common. Other hormones are derived from common precursors found in plants, such as the animal retinoids and prostaglandins and the plant growth regulators abscisic acid and jasmonate. The use of common precursors certainly constrains the overall structure of hormones, but later synthetic steps are also often conserved to some degree, suggesting that the final structures have some inherent properties that make them good signalling molecules, regardless of the organism in which they function.

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