The Rational Argumentator

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Evolutionary theory can explain a variety of interesting developments among the higher animals, including internal temperature regulation, the emergence of Chordata – the phylum to which humans, too, belong – and the origins of advanced modes of feeding.

Internal Temperature Regulation

Given the divergence of both mammals and birds from an ectothermic reptilian ancestor, the ability to internally regulate body temperature must have evolved independently at least twice. Perhaps this is due to convergent evolution, as both birds and mammals frequently expose themselves to cold climates from which there is no escape and in which they must operate.

For example, a bird in flight encounters cold temperatures due to high altitude, while many species of mammals dwell in places where weather changes dramatically with a shift of the seasons. Rather than rely on an external refuge that is not always present, these organisms have mechanisms to preserve a climate favorable to functioning within their own bodies and can thus endure a greater temperature range than reptiles.

The Evolution of Chordates

Tunicates are among the most elementary members of the phylum Chordata; they and organisms like them emerged before the higher animals. Examining the characteristics of tunicates helps understand the beginnings of chordate evolution.

Tunicate larvae possess typical chordate features, including a notochord, pharyngeal pouch, and a dorsal tubular nerve cord. The larvae also have gill slits that are present in a more advanced form in higher chordates. In the adult, the notochord and nerve cord seem to have transformed themselves into the nerve ganglion, while the pharyngeal gill slits have been enlarged. In the latter respect, the adult tunicate still exhibits a prominent characteristic of chordates, though the other two distinguishing features have become less apparent.

Tunicate larvae resemble lancelets more closely than adult tunicates. This further reinforces the idea that the essential characteristics of the chordate phylum became essential parts of adult organisms through paedogenesis. If an ancestral form of larva obtained sexual maturity prior to metamorphosis, it would possess the evolutionary advantages of mobility (as opposed to the sessile existence of tunicates) and a streamlined shape. This may have led to the evolution of lancelets – the next step up in the phylum Chordata in terms of complexity.

The Evolution of Advanced Modes of Feeding

The evolution of more advanced modes of feeding also occurred as more complex species developed. Tunicates are sessile, semi-passive suspension feeders, while the more active lancelets actively scour the seas for tiny bits of food trapped within the water. The jawless and rather primitive lamprey cannot immediately kill and consume its prey; its lack of teeth forces it to gradually suck out the victim’s insides.

With the evolution of the jaw, however, a more effective mode of predation was rendered available, both for those animals that fed on plants and those that consumed smaller animals. While very few agnathan, jawless species survive to this day, jawed orders are amply represented. The birds evolved another jawless yet effective mechanism for obtaining food: the beak. The beak enables a bird to pierce or bite its prey in flight in a speedy manner and to carry it back to the nest.