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Reconstruction of Rhynia The idea that animal and plant species were not constant, but changed over time, was suggested as far back as the 18th century. While it is easy to imagine natural selection producing the variation seen within genera and families, the transmutation between the higher categories was harder to imagine.

With the increasing mapping of the divisions of plants at the beginning of the 20th century, the search began for the ancestor of the vascular plants. The simple form echoes that of the sporophyte of mosses , and it has been shown that Rhynia had an alternation of generations , with a corresponding gametophyte in the form of crowded tufts of diminutive stems only a few millimetres in height.

From a carpet of moss-like gametophytes, the larger Rhynia sporophytes grew much like simple clubmosses, spreading by means of horizontal growing stems growing rhizoids that anchored the plant to the substrate. The unusual mix of moss-like and vascular traits and the extreme structural simplicity of the plant had huge implications for botanical understanding. The term "missing link" refers back to the originally static pre-evolutionary concept of the great chain of being , a deist idea that all existence is linked, from the lowest dirt , through the living kingdoms to angels and finally to God.

Jean-Baptiste Lamarck envisioned that life is generated in the form of the simplest creatures constantly, and then strive towards complexity and perfection i. By that time it was generally thought that the end of the last glacial period marked the first appearance of humanity, but Lyell drew on new findings in his Antiquity of Man to put the origin of human beings much further back in the deep geological past.

Lyell wrote that it remained a profound mystery how the huge gulf between man and beast could be bridged. The single molar was larger than any modern human tooth, but the femur was long and straight, with a knee angle showing that " Java Man " had walked upright. At the time it was hailed by many as the "missing link," helping set the term as primarily used for human fossils, though it is sometimes used for other intermediates, like the dinosaur-bird intermediary Archaeopteryx.

Such jumps can be explained either by macromutation or simply by relatively rapid episodes of gradual evolution by natural selection, since a period of say 10, years barely registers in the fossil record.

While each find will give rise to new gaps in the evolutionary story on each side, the discovery of more and more transitional fossils continues to add to our knowledge of evolutionary transitions. Punctuated equilibrium The theory of punctuated equilibrium developed by Stephen Jay Gould and Niles Eldredge and first presented in [61] is often mistakenly drawn into the discussion of transitional fossils.

These transitions, usually traceable in the same geological outcrop, often show small jumps in morphology between extended periods of morphological stability. To explain these jumps, Gould and Eldredge envisaged comparatively long periods of genetic stability separated by periods of rapid evolution. Gould made the following observation concerning creationist misuse of his work to deny the existence of transitional fossils: Since we proposed punctuated equilibria to explain trends, it is infuriating to be quoted again and again by creationists —whether through design or stupidity, I do not know—as admitting that the fossil record includes no transitional forms.

The punctuations occur at the level of species; directional trends on the staircase model are rife at the higher level of transitions within major groups.


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