The frog’s great leap forward
What a purposeful transformation the fertilised egg makes into a fully formed organism.
That great Victorian naturalist Thomas Huxley compared it to a master potter moulding the contours of a formless lump of clay: ‘pinching up the head at one end, the tail at the other, fashioning flanks and limbs in due proportion’.
He judged the impression of ‘a hidden artist seeking to perfect his work’ to be ‘the most admirable of all nature’s miracles’.
Almost more admirable still is the transformation of one form of life into another: the humble, earthbound caterpillar metamorphosing into the ethereal butterfly borne aloft on its iridescent wings.
We can have little direct knowledge of such ‘natural miracles’ as they unfold at microscopic level – with one astonishing, if familiar, exception signalled in early spring by the unmistakable sound of frogs croaking in a pond.
Their breeding season has begun. The male, with much splashing and kicking, seizes his partner from behind in the tightest of embraces, pressing down on the eggs within her belly, fertilising them with his sperm as they are ejected.
The female swims off to a quiet part of the pond to recuperate, leaving behind her jelly-like spawn, swollen with water, floating freely.
The spawn’s subsequent development over the next three months is probably the most intensely scrutinised of all complex biological phenomena.
It provides a unique opportunity to observe in real time what is required in becoming first aquatic, herbivorous and fish-like – and then four-legged, carnivorous, air-breathing and terrestrial.
That transformation gives us a sense of what was entailed in that momentous evolutionary event 300 million years ago, when the earliest amphibious forms of life emerged from the primordial seas to colonise dry land.
The curtain rises on this drama when, a couple of weeks after fertilisation, a minute, comma-sized tadpole liberates itself from the jelly-like spawn. Soon it acquires the three features necessary for its transient fish-like existence – external gills with which to breathe; a muscular tail to propel it through the water; and a mouth for feeding, connected to a long, tubular intestine curled within its body like a watchspring.
For the next two months, the tadpole is in essence a feeding machine, growing in size and strength as it absorbs the nutrients from the plants and algae on which it grazes. The only physical change during this time is the emergence of a pair of limb buds on either side of its tail.
The wondrous metamorphic transformation, when it happens, comes all in a rush. No aspect of the tadpole’s being is untouched as, over a few short days, its organs and parts are broken down, refashioned and replaced with entirely new ones.
Externally, the tail shrinks rapidly, its skin, muscle, cartilage and nerves dissolving away. The cellular material is swept into the bloodstream. It becomes the building blocks of a much-enlarged skull, a widely articulated jaw and that powerful muscular tongue for capturing the insects, worms and beetles to satisfy its now carnivorous diet.
The tadpole’s small, sideways-directed eyes grow large and bulging. They are kept moist by newly formed tear glands and swivelled by a set of muscles that confer a 360-degree field of vision. Meanwhile, the limb buds elongate to form those characteristic, long, spindly yet powerful legs – the frog’s most striking and glorious attribute. They endow a jumping prowess unparalleled in the animal kingdom.
Internally, the long tubular gut degenerates, reassembling itself into the several specialised structures identical to our own: an acid-secreting stomach, duodenum, small intestine and an ascending and descending colon.
And, most significant of all, the circulatory system is remodelled with a three-chambered heart and maturing lungs, in anticipation of the frog’s imminent air-breathing existence.
And so it is that the pond – at one moment brimming with tadpoles flitting here and there – is, a week later, almost deserted as, reincarnated as frogs, they leap to dry land.
Despite the many insights offered by the frog’s life cycle into the complexities of embryology, the fundamental question it poses remains as irresoluble as ever.
How does the same set of genes give rise to two such profoundly different beings? How does it orchestrate their transformation, one into the other, while recapitulating, in a few days, the profound anatomical and physiological changes that took their earliest amphibian ancestors several million years to accomplish?