The transition from water-based living to land is one of the most pivotal moments in the history of life on Earth. A number of fossils, discovered by a team of researchers in the Canadian Arctic, sheds light on this crucial question. In 2004, a team of researchers lead by Neil Shubin from the University of Chicago, discovered a series of fossils which help answer the questions surrounding this transition.
Tiktaalik – from fish to land-dwelling vertebrates
The fossils were discovered on Ellesmere Island, northern Canada, in 2004. Named Tiktaalik roseae, the genus name meaning “large shallow water fish” in the indigenous language of Canada’s Nunavut Territory nations, is a transitional extinct species possessing features of fish and also tetrapods – four-limbed vertebrates. Tiktaalik used its frontal fins to move itself in a walking fashion in shallow waters, straddling the transition to amphibious living.
Tetrapods are four-limbed vertebrates – amphibians, reptiles, mammals, me, you – it includes an extensive series of animal phyla. This body form first appeared in the Devonian geologic period. The Devonian is known by palaeontologists as the age of fishes. Amphibious animals, emerging in the Devonian, came to be the dominant form of life in the next geologic period, the Carboniferous.
Tiktaalik demonstrates the transition from swimming fish to land-based locomotion for vertebrates. Tiktaalik, while possessing fish characteristics, also had wrist bones, so it could propel itself with its front limbs. Wrist bones are lacking in fossils earlier than the Tiktaalik. Located in sediment beds dating back to 375 million years ago, this stratigraphy layer is located in Devonian geological period.
Qikiqtania – the ‘fishapod’ which went back to the water
The amphibians, the first true tetrapods, evolved from the lobe-finned fishes, but finding the transition from the fishy ancestors of amphibians and true tetrapods has been challenging until now. Related to the discovery of Tiktaalik is another fossil cousin, Qikiqtania wakei, named from the indigenous Inuit languages where the fossil was found.
Another ‘fishapod’ – no, that word is not a scientific classification, but a portmanteau made up by writers. Actually, Qikiqtania is a type of elpistostegalian, a prehistoric species of lobe-finned fish. Qikiqtania’s pectoral fin contains a humerus bone. However, Qikiqtania was more suited to life in the water, and returned there soon after its land-dwelling phase. Neil Shubin, a palaeontologist from the University of Chicago, stated about Qikiqtania that:
The specimen includes partial upper and lower jaws, portions of the neck, and scales,”
“Mostly importantly, it also features a complete pectoral fin with a distinct humerus bone that lacks the ridges that would indicate where muscles and joints would be on a limb geared toward walking on land.”
The media release about these findings from the University of Chicago make the following important point:
We tend to think animals evolved in a straight line that connects their prehistoric forms to some living creature today, but Qikiqtania shows that some animals stayed on a different path that ultimately didn’t work out. Maybe that’s a lesson for those wishing Tiktaalik had stayed in the water with it.
While Tiktaalik’s front fins contained bones which correspond to our humerus, wrist, ulna and radius bones, the later Qikiqtania only had a corresponding humerus bone. Qikiqtania, while closely related to Tiktaalik, took on a different evolutionary pathway. Tiktaalik, in contrast to Qikiqtania, had a mobile neck, allowing it to support its head out of the water and adjust to gravity. The fish-to-tetrapod transition marked the beginning of the vertebrate dwelling in terrestrial ecosystems.
Nowhere in the scientific literature is there any reference to a ‘social Darwinist’ competition between emergent species, nor any ‘struggle for existence’. The tetrapods did not emerge by smashing their competitors, or becoming the strongest ‘king’ of their ecosystem. Neither is there any reference to a supernatural creator, or teleological direction in the evolutionary process. If you want to discuss philosophical issues of theism, or a faith-based natural history of life on Earth, please save that for another blog article.
Evolution, rather than proceeding in a straight linear fashion, moves in a series of branching tree-like pathways. Tiktaalik, and Qikiqtania, are not merely ‘stepping stones’ on the way to the eventual emergence of vertebrate organisms.