Missing links and a look at the Romer’s gap
The term “missing link” was coined in Darwin’s time and was used to denote the “hypothetical organisms that linked different groups, and especially humans with anthropoid apes.” Perhaps the most famous “missing links” are those linking humans to our more ape-like ancestors. The term “missing link” is misleading, however, because it supposes that all creatures are linked in a hierarchy or Chain of Being when the case is rather that all creatures share common ancestors. The term also implies that there are certain links that are missing and thus disprove the theory of evolution. Modern scientists speak of transitional forms instead of “missing links.”
Cotner and Moore define a transitional form as: “An organism having anatomical features intermediate between those of two major groups of organisms in an evolutionary sequence. Transitional forms show evolutionary sequences between lineages by having characteristics of ancestral and newer lineages. Since all populations are in evolutionary transition, a transitional form represents a particular evolutionary stage that is recognized in hindsight.” Why don’t we find something halfway between a hippo and a whale, then? Miller explains that transitional fossils are retrospective: “For example, transitional forms are not to be found between living whales and their closest living relatives the hippos, but between whales and their common ancestors with the hippos. Such forms will be unlike anything living today. Transitional forms are found by moving down the tree of life into the past, not by trying to jump from limb to limb.”
Keith B. Miller addresses two errors with regard to the fossil record and missing links: “There are two opposite errors that need to be countered about the fossil record: (1) that it is so incomplete as to be of no value in interpreting patterns and trends in the history of life, and (2) that it is so good that we should expect a relatively complete record of the details of evolutionary transitions within most or all lineages.”
David H. Bailey states that many of the gaps pointed out by creationists have been filled over the past few decades. Dawkins states that “for a large number of fossils, a good cause can be made that every one of them is an intermediate between something and something else.” It is true that there are gaps in the fossil record, but that is exactly what one would expect if one takes into consideration the difficult process of fossilization and the amount of strata which have been excavated. A rare occurrence known as fossil Lagerstätten took place where one encounters thick layers of fossil-rich rock. These are very rare indeed. Organisms that lack hard parts (eg. Ediacaran organisms, sponges, plants) have very little hope to be fossilized and even when an organism has hard parts, dissolution and recrystallization could erase any traces of fossilization. Erosion and changes in the formation of rock can also destroy fossil evidence. Terrestrial organisms are less likely to be fossilized than those in aquatic environments, because sediments in water increase the chances of fossilization. Even when there are fossil-rich layers, these may be situated in inaccessible places, e.g. on the bottom of the sea or in the subsurface. Long-lived and abundant species stand a bigger chance of being preserved in the fossil record than short-lived or scarce species. For example, a fossil impala would be more probable than a fossil aardvark. As Charles Darwin stated: “The crust of the earth is a vast museum; but the natural collections have been imperfectly made, and only at long intervals of time.”
One famous gap in the fossil record is called Romer’s gap and spans from the end of the Devonian period (about 360 million years ago) and the early Carboniferous period (about 340 million years ago). This gap basically correlates to the evolution of fish into amphibians. Several of these transitional forms have been discovered:
- Eusthenopteron foordi, discovered in 1881 in a collection of Canadian fossils, is an example from the late Devonian period. At the bases if its fins, this creature had bones which are analogous to those of terrestrial animals. Eusthenopteron looked like a fish and probably spent its life in water.
- Panderichthys rhombolepis is another specimen from the Devonian. It had less fins and thicker ribs than fish. Thicker ribs are needed to support the body when the creature is out of the water, but Panderichthys probably lived mostly in water.
- Ichthyostega stensioei and Acanthostega gunnari looked like fish with legs. Their cranial structure and skeletons resembled that of a fish, but their ribs were even thicker than those of Panderichthys. Both creatures could breathe air. Between the two, Ichthyostega probably spent more time on land than Acanthostega and moved like a seal does on land. Ictheostega would be the first known vertebrate that did not move like a fish. In fish, the hyomandibular bone serves to support the gills. This bone corresponds to the ear bone in mammals, also called the stapes. In Acanthostega the stapes resemble the hyomandibular bone of a fish and could not vibrate, thus rendering it useless for the purposes of hearing. Later, in the Carboniferous period, amphibians possessed a hyomandibular bone that could be used for hearing.
- Tiktaalik roseae was discovered in 2004 and had amphibian-like skull, neck, ribs, elbows, wrists, and fingers, yet had fins, scales, and gills like a fish. It could probably prop itself up using the elbows, wrists, and fingers. Tiktaalik did not have gill plates and could thus move its head from side to side.
Also see: http://waitbutwhy.com/2013/12/your-ancestor-is-jellyfish.html for a cute family history
Sources:
Bailey, D H 2010. Creationism and intelligent design: Scientific and theological difficulties. Dialogue: A journal of Mormon Thought 43/3, 62-81.
Cotner, S & Moore, R 2011. Arguing for evolution: An encyclopedia for understanding science. Greenwood: Santa Barbara.
Daintith, J & Martin, E (eds.) 2010. Oxford dictionary of science. Oxford: Oxford University Press.
Darwin, C 1909. Origin of species. New York: P F Collier and Son.
Dawkins, R 2009. The greatest show on earth: The evidence for evolution. London: Bantam Press.
Miller, K B 2003. Common descent, transitional forms, and the fossil record, in Miller, K B (ed.) Perspectives on an evolving creation. Grand Rapids: William B Eerdmans.152-181.
Rice, S A 2007. Encyclopedia of evolution. New York: Facts on File.
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