Tiktaalik roseae (artists impression)
Yet the childish superstition persists in the more backward and scientifically illiterate parts of the world, despite evidence of so many transitional species showing a clear evolutionary change from an earlier taxon to a later one. One such transition that has evidence in the fossil record is the evolutionary transition from lobe-finned fish to terrestrial vertebrates.
A key stage in the development of the terrestrial vertebrates was the transition onto land of air-breathing fish, so a key piece of conditioned denialism for Creationists is to claim there are no fossils showing this transition because their dogma dictates that there are no transitional fossils, for no better reason than Darwin said there should be some.
In fact, of course, there are very many of these fossils showing intermediate species, capable of crawling out onto land and of living in water.
I described this transition, and the ecological changes that facilitated it, in my book, What Makes You So Special: From the Big Bang to You, which deals with the science behind the formation of the Universe, of Earth and of life on Earth, and eventually to humans and the development of human cultures and civilisations, culminating ultimately in you, as a unique individual and beneficiary of that immense chain of causality. So, it's good to know this subject has now been made into a YouTube video, which entirely vindicates what I said: Now, here is my description:
The earliest fish came in two basic types, both jawless – an eel–like creature similar to the lampreys and hagfish, and a group of small, armoured fish, the placoderms. It was one of this latter group that first evolved a jaw, probably from one of its gill arches. It was the jawed fish that diverged into two major groups – the cartilaginous fish, or Chondrichthyes, and the bony fish, or Osteichthyes. The bony fish in turn diversified quickly into the ray–finned fish, or Actinopterygii, and the lobe–finned fish, or Sarcopterygii. It was from these lobe–finned fish that our ancestors – the first terrestrial tetrapods – evolved. One group of lobe–finned fish are the coelacanths. This means we share a more recent common ancestor with them than either of us share with the sharks, skates and rays. The basic quadruped limb plan was based on structures evolved in these lobe fins. The basic plan is a single long upper bone, jointed to two long lower bones then an arrangement of bones in the writs and ankle, then a hand or foot with a variable number of digits which settled down to a maximum of five. This is modified according to need, especially in the number and length of digits. The lower ‘leg’ of a horse, for example, is really a single digit. Even such highly modified limbs as the whale flipper and the wings of bats, birds and the extinct pterodactyls are modifications of this basic plan.
From that small, un–remarkable member of the Cambrian biota; those stiff little chordates that swam around amongst the trilobites and fearsome Anomalocaris with its huge, jaw–like appendages that were once thought to be a different species in their own right, big things were coming; in fact, the biggest things to ever live – the blue whale. From that unpromising beginning were to evolve the fastest running animal, the fastest flying animal, birds that can soar on the wind and sing symphonies, three forms of powered flight, echolocation at least twice, a brain capable of doing calculus without realising it, and an ape that can go to the moon.
But first, it had to learn to walk on land and breathe air.
To walk on land there had to be survival advantage in doing so. It had to give access to more food, or sanctuary from predators. The probability is that what gave the first fish an advantage when they came out onto land was the same thing that gives modern mudskippers an advantage – lots of small arthropods and other terrestrial animal life to eat.
The earliest known land invertebrate was a species of millipede known as Pneumodesmus newmani that lived about 428 million years ago. It was found by an amateur palaeontologist and bus driver from Scotland named Mike Newman, hence its specific name. It is important to evolutionary biologists in that it has numerous holes in its cuticle that are probably spiracles. These are the tubes which lead into the bodies of terrestrial arthropods through which oxygen is taken and carbon dioxide is expelled. These spiracles are a strong clue that it lived on land and breathed air.
Before arthropods such as P. newmani could move onto land there needed to be some sort of ecosystem for them to move into. There would have been precious few advantages crawling about on bare rocks or sand, and soil needs plant matter, bacteria and fungi.
Evolution and ecology are really aspects of the same thing, because ecology is about how species adapt and interrelate to form a complete system of interdependent organisms. Without evolution, none of the interdependent organism could fit into their particular niche and compete with other organisms for resources. So, evolution created ecosystems and ecosystems drive evolution. But in the early days of life on Earth, before life emerged from the oceans where it had first evolved and diversified, there was no ecosystem as such on land, so how did the first emerging life manage to eke out an existence and what reason did it have to go there in the first place?
In 2016, scientists believe they found evidence (Jinzhuang, et al., 2016) that an early plant, related to the club mosses, may have created its own ecosystem by interaction between its rhizomes and sediment from silt-laden floods. Rhizomes are underground stems by which many plants spread to form large colonies. Plants like this could have lived in shallow water prone to frequent drying so rather than the plants leaving the water, the water left the plants high and dry at times. It is but a small step then to spread out into 'dry' land prone to frequent flooding.
So, our intrepid millipede had some reason to move onto land in the form of plants to eat, as did others, followed no doubt by predators. Together with the early plants and maybe starting in estuarine tidal mudflats where there was an advantage in being able to survive dry periods, they created soils and ecosystems and niches for other creatures to emerge from the sea into or to evolve in situ from earlier colonists.
And one of these was our ancestor, an advanced lobe–finned fish that had evolved limbs with which to crawl around on the bottom of the sea where its body weight would have been partially supported by water. The other thing it had evolved was the ability to breath out of water. This was maybe not as big a step as it might intuitively seem and breathing air did not require the fish to live out of water.
The early fish, probably the armour–plated placoderms, had evolved the swim bladder as an aid to buoyancy from an outgrowth from the gut. As so often with evolution which has no hesitation to use a structure evolved for one purpose for something completely different, this swim bladder was probably co–opted to act as an oxygen store by inflating it with air gulped from the surface. In the early Devonian, oxygen levels were only about half of today’s value, but a given volume of air contains much more oxygen than the same volume of water so anything that could access this atmospheric oxygen would have been at an advantage. It would have been especially advantageous for a lobe–finned fish that crawled about the bottom, foraging in the deoxygenated silt.
So, any fish that could haul itself out of water and ‘breathe’ atmospheric oxygen, even for a short time, could also eat the terrestrial invertebrates that had got there first, and those that could do it better and for longer and which could move faster would do better than the slower ones still tied to the water and unable to venture far from it.
Despite the creationist mantra that there are no transitional fossils, there are actually a large number of fossils showing the progress of the lobe–finned fish into terrestrial tetrapods. One of the best-known examples is Tiktaalik roseae, the discovery of which represents a successful prediction of the Theory of Evolution. It was discovered by Edward Daeschler of the Academy of Natural Sciences in Philadelphia, Neil Shubin of the University of Chicago, and Farish Jenkins of Harvard University in Cambridge, Massachusetts. They had worked out that there should be a transitional species between the lobe–finned fish and the terrestrial tetrapods (amphibian, reptiles, birds and mammals) and that those fossils should be found in Devonian rocks.
Noticing that just such exposed rocks were to be found on Ellesmere Island, in Northern Canada, they assembled a team to go and look for them. Because of their remoteness and inaccessibility, only being accessible by plane, the rocks had never before been explored for vertebrate fossils. They found several well–preserved specimens of exactly what the theory predicted should be there; Tiktaalik, given an Inuit name for a burbot – a freshwater relative of the cod – suggested by Inuit elders in Canada’s Nunavut Territory, which includes Ellesmere Island.
The apparent gap in the fossil record between the lobe–finned fish and terrestrial tetrapods is still referred to as Romer’s Gap, named after the palaeontologist Alfred Romer who first recognised it. It was substantially closed and shown to be more due to a lack of discovered fossils rather than an actual lack of them, by a team from Canada and the United Kingdom (Anderson, Smithson, Mansky, Mayer, & Clark, 2015).
The fossil record is never better than the latest discovery. A detailed examination of rock deposits at Horton Bluff, lying along the Avon River near Hantsport, Nova Scotia, Canada, showed abundant fragments of intermediate fossils showing both features of lobe–finned fish and the basic skeletal pattern of terrestrial tetrapod limbs. They differed in the number of digits, showing that the five–toed basic pattern of the general tetrapod limb had not yet evolved.
If you could ever get a creationist to describe what they would expect a transitional species midway between a lobe–finned fish and a salamander to look like – which is about as easy as drawing blood from a stone – they would be hard–pressed to describe something that was not a lot like Tiktaalik or the fossil remains found at Horton Bluff. Another gap in the story is closing fast.
So, our ancestors learned to live on land, to breathe air and to walk on four legs and we left our fish cousins to evolve on their own branch of the tree of life as we branched out on our own. The world of dry land where plants and invertebrates were already evolving and diversify lay before us, full of opportunity. First though, our ancestors were still tied to the water and wet places because their skins needed to be kept moist and for another very good reason – reproduction.
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