Comparative Embryology and its Evolution on Multiple Species

“Fish, mammals and reptiles are all very different types of animals but they all started from the same roots and that is why some people fine comparative embryology so interesting.”

— Johan

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Comparative Embryology and Its Evolution onAncient Species

Johan

_____Johan_____

Comparative embryology is the comparison of embryo development across species. (​education.seattlepi.com​) Every organism on earth is related in some way, but evolution has allowed us to grow and become unique from one another. That is why embryos start off in the same state, but slowly start to get unique characteristics over time. Fish, mammals, and reptiles are all very different types of animals, but they all started from the same roots and that is why so many people find Comparative embryology so interesting. It shows us how all these animals were once the same but changed over time. The first known study of comparative embryology was actually by Aristotle in 400 BC back in ancient Athens. He would crack animal eggs and study them closely. He actually discovered the two major cell division patterns that embryos are formed in. Aristotle was extremely erudite at a time where education was minimal. But after his death, it was not till the 1600s till scientists continued these studies. It kickstarted in 1651 when William Harvey discovered that every animal, even

those who give birth, originated from eggs, among other findings. This was also around the time the microscope was invented and that is when comparative embryology really started becoming a popular subject. One of the most famous scientists in this field was Karl Ernst von Baer, who made the four principles of comparative embryology (See below). Anyway with some background information out of the way let's take a look at some examples of comparative embryology. First of all we have the mouse, which has a gestation period of around 20 days depending on the species (Gestation meaning reproduction). It starts off as 1 cell but quickly gets more advanced. By day 8 it will start developing the heart, neural, and limbs. And by day 9 its poster will form

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_____Johan_____

In comparison, the chicken has a gestation period of 10 days - 14 days. And it has also developed limbs and posters around the same time.

And that is why by days 7-10 the chicken will start to develop characteristics of its own such as a beak, new changes to its skin, growing claws and feathers, and other features that make it unique from other species.

Concurrently the rat has started getting characteristics of its own. 17 days in the rat will have grown a large tail, fingers, toes, whiskers, and other things that differentiates it to other species like the chicken.

Further Development

At this time both embryos look pretty similar, right? Well, that is because, according to Karl’s Ernst second law, all vertebrates (A large group of animals categorized by their backbone structure) will have the same type of skin and general features. For example, even if we do not use them past birth, human embryos still have tails because we shared all those general features. And we all come from eggs, and we all start off as a 1 singular cell. But according to Karl’s 4th law, even if we originally share characteristics with other embryos, later we start to be diverse and therefore, nobody goes through the same stages as others.

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_____Johan_____

These two creatures started off similarly but as time goes they just get more distinct and by the time the gestation period has ended, they look nothing alike.

Evolution o� Comparative Embryolo�y: Now vs Then

These two have evolved for centuries upon centuries to become what they are today, but let's take a look at where they came from and how these life forms changed throughout the history of life on Earth. This part will focus on the snake and which you probably already know how it looks like.

According to intrestingenginering.com a year ago a “95-million-year-old skull fossil points to biblical snakes with hind legs'' and according to the article it was discovered in Argentina. Scientists managed to piece together these fossils, and by using today's technology, they figured out that at some point it looked like this (Picture below) As you can see it is clearly bigger than your average snake and it has little hind legs that it uses to walk around. Since the fossils were mostly intact they managed to piece out that these snakes had large mouths, sharp teeth and according to the article they “​fed on larger prey than previously believed and that they were very similar to our current day big-bodied, and big-headed lizards, such as the Komodo dragons.” With the use of comparative embryology, these lizards evolved from having huge heads and sharp teeth to the little snakes that are found in the wild today. But bigger is not always better and that is probably why they evolved to become much smaller. They might not have been as fearsome as they looked, and therefore they started eating less food, getting fewer calories, and then they eventually evolved to become much smaller. And today most snakes have gotten rid of those teeth entirely.

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_____Johan_____

Works Cited

Becker, Andrea. “What Is Comparative Embryology?” ​Education​, 21 Nov. 2017, education.seattlepi.com/comparative-embryology-6455.html.

Gilbert SF. Developmental Biology. 6th edition. Sunderland (MA): Sinauer Associates; 2000. Comparative Embryology.​ Available from: ​https://www.ncbi.nlm.nih.gov/books/NBK9974/

Roberts, M. B., & King, T. J. (1987). ​Biology: A functional approach. Students' manual​. Nelson Thornes.

“Animal Development.” Embryology, embryology.med.unsw.edu.au/embryology/index.php/Animal_Development.

Lang, Fabienne. “95-Million-Year-Old Skull Fossil Points to 'Biblical Snake' with Hind Legs.” Interesting Engineering​, Interesting Engineering, 22 Nov. 2019,

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interestingengineering.com/95-million-year-old-skull-fossil-points-to-biblical-snake-with-hindlegs.

Pictures:

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“Mouse Timeline Detailed.” ​Embryology​, embryology.med.unsw.edu.au/embryology/index.php/Mouse_Timeline_Detailed.

Chicken Embryo​, ​www.microscopy-uk.org.uk/mag/artnov04macro/mlchicken.html.

“Baby Mouse (с Изображениями): Мышонок, Милые Детеныши Животных, Домашние Крысы.” ​Pinterest​, www.pinterest.com/pin/155726099584667238/.

“A Newborn Baby Chicken Standing on the Ground.” ​123RF​, www.123rf.com/photo_101444443_a-newborn-baby-chicken-standing-on-the-ground.html.

Cambridge University (Jan 23 2012). ​Mouse embryo developing over time. https://www.youtube.com/watch?v=G7H2mEQYIWc

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