Every sexually reproducing creature begins as a unicellular zygote and grows into an adult through cell division and differentiation. The study of an organism's early development is known as embryology. Let us now look at the evidence for evolution provided by comparative embryology.
Aristotle was the first known individual to study embryos. He described how animals hatched in eggs (oviparously) and, via live birth (viviparously), evolved differently by observing embryos of various species. He identified two types of egg cell division: holoblastic, in which the entire egg divides and becomes the creature, and meroblastic, in which only a portion divides and becomes the creature. The invention of the microscope resulted in further discoveries in comparative embryology. Since then, many others have contributed to the area, including Ernst Haeckel and Charles Darwin.
Many false notions were developed throughout the early years of comparative embryology. For example, German scientist and philosopher Ernst Haeckel believed all species underwent a "re-run" of evolution, claiming that 'ontogeny repeats phylogeny' during development. To become a mammal, an embryo, according to Haeckel, must begin as a single-celled creature, then evolve into a fish, then an amphibian, a reptile, and lastly, a mammal. The notion was widely accepted, only to be debunked several years later.
The German anatomist Ernst Haeckel proposed the recapitulation hypothesis or biogenetic law, which provided embryological evidence for organic evolution. According to the law, ontogeny recapitulates phylogeny. This suggests that an organism's racial past is remembered or recalled during its developmental history. This means that embryos, during their development, reenact the evolutionary history of their forefathers in a shortened version. Haeckel argued that distinct phases in an individual's early development (ontogeny) symbolise the many forms and kinds through which the race or species travelled (phylogeny) during evolution and give a direct proof of its line of ancestry.
In successive moults, the larvae go through six different phases. Each larval stage in this group closely resembles the adults, progressing from the primitive to the mature form. Primitive crustacean larvae stop growing and differentiating early in this sequence, whereas advanced crustaceans exhibit all or most phases.
In another case, all vertebrates' kidneys emerge from the nephrotomy, a segmented mass of mesoderm. Among vertebrates, however, there are three unique types of kidneys: pronephros, mesonephros, and metanephros, which develop one after the other during growth and development. The first kind is replaced by the second, replaced by the third. All vertebrate embryos begin with a pronephric kidney that only uses the anteriormost section of the nephrotomy.
Only a few bony fishes and hagfishes preserve this pronephric kidney as a functioning adult kidney. Most other adult fishes, amphibians, and embryos of reptiles, birds, and mammals progress to the mesonephric stage of kidney development.
Adult fishes and amphibians have functioning mesonephric kidneys. Finally, kidney development progresses to the metanephros stage in reptiles, birds, and mammals, the adult organism's functioning organ. These findings imply that higher vertebrates developed from lesser vertebrates through the development of various features.
Let us now examine the embryos of several animals. At various stages of development, the embryos of many vertebrates show a high degree of resemblance. They only differ morphologically in the latter phases of development and differentiation. Plants' embryology is simpler. Therefore, recapitulation is less obvious, yet there are few notable instances.
The Acacia tree leaves are very complex, yet its seedlings have simple leaves like their progenitors. The Eucalyptus is another beautiful example that you should be familiar with. Adult tree leaves are narrow blades twisted to expose the thin edge to the sun. However, the seedlings' leaves are broad and orientated like those of more recognisable trees. Conifers have revealed an intriguing and likely analogous phenomenon. If a conifer is harmed and the lesion can heal, the new growth may be histologically different from normal tissue. The novel tissue has a structure similar to that found in Mesozoic-era conifers.
Haeckel's biogenetic rule was heavily criticised since there is no accurate phylogenetic recapitulation during ontogeny. This is because the similarities observed in ontogeny are predominantly between embryos of related species rather than between embryos and adult relatives. Only on rare occasions do they allude to grownups. Thus, ontogeny merely recapitulates phylogeny in the sense that ancestral embryonic phases are replicated, and even these can be radically altered by natural selection-favoured modifications.
According to the recapitulation idea, embryos must repeat the past, compressing certain phases and deleting others, without changing to the embryonic way of existence. The truth is considerably different; embryos, like adults, must adapt to survive in a challenging environment. Differences in cleavage pattern in different embryos, for example, are related to the quantity of yolk in the eggs. This is a fundamental biologically important adaptive characteristic.
Consider egg adaptations for development in fresh water, salt water, or terrestrial settings. There are considerable variations between several families of echinoderms that can be attributed to embryonic adaptations. Many scientists believe these changes result from the substantial natural selection of larvae since their split from their progenitors.
Haeckel's observations were founded on von Baer's ideas of embryonic differentiation, which are as follows −
During embryological development, the appearance of traits typical of the particular phylogenetic line ('generic' characters) comes before those typical of the species itself ('special' features).
During development, there is a shift from the more general to the less general and eventually to the specific characters.
During its evolution, an animal gradually deviates from the morphologies of other related species.
The embryonic phases of lesser animals mirror those of young animals but not those of adults.
Von Baer's embryonic differentiation principles offer a better guide to interpreting embryological information that supports organic evolution.
Comparative embryology provides evidence for evolution, with Aristotle being the first to study embryos and Ernst Haeckel proposing the recapitulation hypothesis. Haeckel's law suggests that embryos reenact the evolutionary history of their forefathers in a shortened version. Vertebrate embryos begin with a pronephric kidney, progress to the mesonephric stage, and then to the metanephros stage, suggesting that higher vertebrates developed from lesser vertebrates through the development of features.
Ontogeny merely recapitulates phylogeny, but embryos must adapt to survive in a challenging environment. Von Baer's embryonic differentiation principles offer a better guide to interpreting embryological information.
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