At the pond’s edge, something stirs. The mist of frogspawn that has hung in the water for weeks is coming to life. Not so long ago only the sharpest eye could have picked out the tiny, dark pinpoints at the centre of each egg.

But now distinct shapes are emerging and some are twitching in their eagerness to escape their gelatinous nursery. Each frog embryo is, in essence, a multiplying, self-organising complex of cells held within a nourishing and protecting egg. Yet, as they develop into independent living creatures, these humble frogs reveal the astounding ingenuity of life as ably as any so-called higher organism.

The entire odyssey is guided by a set of genetic instructions set down by their parents and carried within every cell. Evolution has honed them so that each element of the developing frog is directed on how to behave at the prompting of its neighbours and the environment around it. It is not simply a matter of pulling the pieces together into the right shape ready to hop towards its destiny. Making a living organism is a process rather than a product.

The infants cradled within the frogspawn look distinctly unfroglike. Indeed, the tadpoles that hatch into the water will be very different from adult frogs. They breathe through gills like fish and swim with a tail. Their rasping mouths scrape algae from the bottom of the pond and they digest their food in a long, coiled stomach.

For some months - it varies from species to species - the tadpoles will disperse through a world that they appear perfectly adapted to. Then, at some carefully chosen signal - part genetic, part environmental - a sudden release of hormones will unleash profound forces that will utterly transform the creatures in a matter of weeks.

Genes that have sat idle in each tadpole flicker into life. Others go quiet while many find new tasks awaiting them. Special cells within the tadpole’s body chomp into organs and limbs that are no longer wanted while others set about building new ones.

Tails and gills are absorbed while legs and lungs grow in their place. The whole skeleton gets revamped. The rasping mouth is rebuilt to make room for a powerful jaw while the gut shortens ready for the carnivorous diet the frog will soon be enjoying.

When a young frog subsequently leaps on to the land for the first time, breathing comfortably in the air and searching hungrily for insects to feast upon, the aquatic, algae-fuelled tadpole it once was is nowhere to be seen. The adult is now ready to find a mate, reproduce and start the whole cycle again.

Metamorphosis allows an organism to match itself better to the challenges that different stages of its life will demand - the tadpole to spread the species, the frog to reproduce it. Organisms such as humans lost that ability millions of years ago as their complexity made the task of a rebuild unfeasible.

Steve Farrar

Weblinks

Metamorphosis explained: http:www.seagrant.wisc.edumadisonjason10metamorphosis.html Frog facts, games and jokes: http:www.ukplus.co.ukukplusclickcounter.jsp?id=43776 amp;brand=UKPlusamp;search=frog+metamorphosisamp;pg=0amp;location Nature’s body changers: http:www.pbs.orgwnetnaturebodychangers British Herpetological Society: http:www.thebhs.org Steve Farrar is science correspondent for The Times Higher Education Supplement