Missing Link

from Wikipedia, the free encyclopedia

A Missing Link ( English for "missing link") is an undiscovered fossil transitional form between evolutionary ancestors and descendants, because of evolutionary theory has been predicted considerations and the tradition gap in the fossil record would close. Such a connecting find has the character of a mosaic , i. H. the fossil shows features of both the older and the younger form. The term Connecting Link is now preferred for this. In English, the term missing link is often retained in the non-academic area. It originally referred to the lack of an intermediate form in the evolutionary series from ape-like ancestors to humans ( hominization ).

Historical

The English geologist Charles Lyell first used the term Missing Link in 1851 to discuss missing stratum members in historical geology . In today's understanding - for suspected transition species between chronologically separable taxa - Missing Link appears for the first time in his work Geological Evidences of the Antiquity of Man from 1863. Previously, William Hopkins had also applied the term to fossil living beings in 1860 , while Charles Darwin still spoke of Transitional Fossils in his groundbreaking work On the Origin of Species (1859) . In the following years Darwin, Thomas Henry Huxley and the German Ernst Haeckel took over the term Missing Link for the presumed missing link between apes and humans .

Initially, missing links were often seen as a counter-argument to Darwin's theory; even today they are sometimes misrepresented that way. From a scientific point of view, however, they have developed into a success story: with well over 1000 former missing links that have been found to date, the theory of evolution has proven itself convincingly.

One of the most spectacular finds was the discovery in the 19th century of the ancient bird Archeopteryx , which combines dinosaur and bird characteristics. A construction plan modification made such a big change in the way of life possible that the lineage of the emerging birds developed explosively. In retrospect, few fossils have survived from this short time, so that at first it appeared that the primeval bird had appeared spontaneously with many changes at the same time. Nevertheless, it is now possible to trace the development of feathers back to ground-dwelling dinosaurs using fossils.

Even fish like Acanthostega (a predecessor of amphibians ) or snail Neopilina galatheae (a link between mollusks and annelids ) are mosaic forms and prove that in the context of macroevolution are many smooth transitions between different groups of organisms.

Conditions Affecting Fossilization

Discoveries of missing links predicted by evolutionary theory have been and are rarely found in practice. This results from various circumstances:

  • The fossilization runs over the geological periods cumulatively, and overlaid organisms after numerous specific conditions that are derived from geological, biological and random circumstances.
  • The mosaic forms between taxa usually only exist for a very short time, as the split lines change physiologically rapidly. This is particularly so when an important characteristic of the new lineage enables far-reaching changes in the way of life, since other characteristics then follow very quickly and convert the blueprint into the form known later. Therefore, especially in the splitting off of new lineages and more modern building plans, only short periods of time are available for the fossilization of individuals.
  • Geologically , sediments must be present that allow fossils to be embedded. This is the case, for example, in alluvial land. But not in mountainous regions, as the geological substrate is primarily removed there.
  • The fate of the resulting sediments is also geologically determined . If they get too deep over time, the rocks are transformed ( metamorphic rocks ) and all fossils are lost. Even magmatites such as granite and basalt cannot contain fossils.
  • Biological conditions are that the individuals after their death are not discovered, eaten and scattered by other contemporaries. This is very different for each animal species and living world .
  • Some species or groups are unlikely to fossilize because they consist only of soft tissue.
  • Many fossils were destroyed by erosion and tectonic movements.
  • Most fossils are only available as fragments.
  • When environmental conditions change, the population of a species is severely depleted. Therefore, there is little chance that the evolutionary changes caused by these new conditions will fossilize.
  • Most fossils only retain features of their external form but little about how the organism functioned.
  • If we compare today's biodiversity with the fossil record, it suggests that the fossils discovered so far represent only a small fraction of the large number of species that have lived in the past.

Current status

There are still numerous forms of evolution, of which no fossil has yet been discovered. However, due to the further development of the theory of evolution compared to the 19th century, the term missing link has lost all explosiveness within evolutionary biology . While the validity of the theory of descent was still controversial at the time it was first interpreted as "transitional fossils " , biologists today trust that the evidence sought will emerge over time and with the ever-increasing number of fossils. In particular, the recognition that evolution does not necessarily always take place gradually and at a constant speed, but can sometimes also take place very quickly, has reduced the importance of missing links . There are also genetic methods of determining kinship, which are increasingly coming to the fore.

Genetics and Missing Links

The fossil missing links are only a question of paleontology today. In addition, the knowledge of genetics comes to the fore. Here, too, there is a constant search for transition series of mutations . You can use them to determine the relationship between any two recent living beings. The fossil creatures can only be classified indirectly with this.

Language research

Based on Huxley's creation, Missing Link is also used in language research .

See also

Individual evidence

  1. ↑ a b c student boys biology. Bibliographisches Institut & FA Brockhaus AG, Munich 2009, page 372.
  2. ^ Charles Lyell: A Manual of Elementary Geology or, The Ancient Changes of the Earth and its Inhabitants as Illustrated by Geological Monuments. (= Elements of Geology, 3d and entirely revised edition) John Murray, London 1851.
  3. ^ Charles Lyell: Geological Evidences of the Antiquity of Man . Dent & Sons, London 1863 (especially Chapter 22).
  4. ^ William Hopkins: Physical Theories of the Phenomena of Life. In Fraser's Magasines. July 1860, p. 88.