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Trace organs They are those that in some organisms are small and usually without function, but in other organisms they are larger and have a definite function.
The evolutionary importance of these vestigial organs is an indication of a common ancestry. A well-known example of vestigial organ in man is the vermiform appendixsmall, functionless structure that starts from cecum (structure located at the point where the small intestine attaches to the large).
In rodent mammals, the caecum is a well-developed structure in which partially digested food is stored and cellulose, abundant in ingested vegetables, is degraded by the action of specialized bacteria. In some of these animals the caecum is a continuous pocket and in others, like the rabbit, it has a narrower end, called appendix, which corresponds to the human vermiform appendix.
Comparative study of embryology of various vertebrates shows the great similarity of early developmental pattern.
As the embryo develops, individualizing characteristics emerge and similarities diminish. This similarity was also found in the embryonic development of all metazoan animals. In this case, however, the more different the organisms, the shorter the common embryonic period between them.
Any evidence of the presence of organisms that lived in ancient times on Earth is considered fossil. The hard body parts of organisms are those most often conserved in fossilization processes, but there are cases where the soft part of the body is also preserved.
These include frozen fossils such as mammoth found in northern Siberia and insect fossils found in amber. In the latter case, the insects that penetrated the sticky resin, eliminated by the pines, died. The resin hardened into amber, and the insect contained therein was preserved in the details of its structure.
Russian scientists have found a baby mammoth female, an already extinct species, that has been preserved in frozen Siberia soil for 40,000 years.
Insects preserved in Ambar.
Fossil impressions left by organisms that lived in past ages are also considered fossil, such as footprints of extinct animals and imprints of leaves, feathers of extinct birds and the skin surface of dinosaurs.
The importance of studying fossils for evolution lies in the possibility of knowing organisms that lived on Earth in ancient times, under different environmental conditions from those found today, and which may provide evidence of kinship with current species. That is why, fossils are considered important testimonies of evolution.