We are familiar with tethering the word ‘linear progress’ with evolution. The events of flagellated animals developing a worm-like locomotory system or humans who inherited motorised locomotory transport systems viable in water, land, and space have always made us believe that evolution has moved forward. But numerous examples disobey the said assertion, including terrestrial tetrapods (shed their legs and moved back into the water bodies), birds (making their wings reductant developing flippers), and lizards (shedding their legs to evolve into snakes).
Recent developmentary biology believes that evolution does not always follow linear progress with time. It follows the best fit direction, which may harmonize the animal best with its biological and physiological activities. Genetics, development, and evolution are the three pivotal factors that drive the route of progressive evolution. The first evidence that describes animals’ walking motion can be traced back to 635-541 millions of years ago, with the centipede-like locomotory development.
Interestingly, various researches claim that the transition of fins to limbs might have initiated from a couple of small genetic mutations.Dr Hawkins’s work beautifully elucidates the mechanistic pathway of how the zebrafish develops a supplementary limb-like skeletal bone, triggered by mutation. In another instance, Thai cavefish have reported having developed a robust pelvic girdle alongside its vertebral column, possibly enabling them to walk and climb.
The assertion might not always indicate that these progressive developments shall provide a perfect analogy of the ancient tetrapod fishes, but should help connect the dots between evolution and mutation, thereby aiding to solve the mystery of the early tetrapodal weight-bearing limb that supported the animals to walk on land.
The examples above define some of the classic progressive evolutionary traits that made animals adapt to a new habitat (for example, change of the habitat from water to land). A recent study found that most of the crocodiles are able to gallop and have been characterized by ancient reptilian traits rather than the latest development. This raises the question, is retro progressive development natural? Similar features may be related to the lizards who lost their legs and developed an elongated body, modifying themselves in the Cretaceous legged snake.
However, the disappearance of the legs was a long process. However, an intermediate form of hind legs and a pelvis has been documented (from fossils) to have persisted for over 70 million years until around 100 million years ago, proving the transition strata’s importance. Around a similar timeline ( 50 million years ago), animals such as whales and dolphins adopted an aquatic lifestyle by dismantling their legs. Several researchers believe that the hippopotamus is also in the transition zone of tuning themselves to opt for a fully aquatic lifestyle.
Indeed, evolution is a complex process, and it selects the routes which favour the animals the most. Whether growing legs or shedding legs is an advantage or acts as a flaw is yet to be understood completely. However, with the advent of computational tools and high-end analysis, tethering the evolution with the ancient species has become more accessible, thereby widening the avenue to rediscover various futuristic insights to the evolutionary trendline.