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This theory is often stated as "Ontogeny recapitulates phylogeny", simply meaning that as an organism develops, it replays its evolutionary history. His theory is closely tied to the figure below (redrawn by Romanes in 1892), which shows striking similarities in the various stages of development of some vertebrates. We now know that Haeckel, perhaps in an attempt to bolster his theory or perhaps because his specimens were incomplete, embellished the drawings to some degree; so these examples must be taken with a grain of salt but hey are correct in a generalized sense. Haeckel, a great believer in the works of Charles Darwin, used this illustration and many of his other drawings to support the theory of evolution and argued that as an individual develops, it repeats the full evolutionary development of its species.
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If Haeckel were to be taken literally, at some point a growing human embryo would be a viable fish, amphibian, reptile, or early primate. But in fact a human embryo can only ever be a human. The commonalities we share with other animals that Haeckel discovered are indicative of our common ancestry and his theory helps us understand that stages in our embryonic development reflect these connections. Some of the weird things human embryos go through include:
• Early on the embryo develops gill slits (more correctly called pharyngeal arches) in its neck. In a human, the first gill bar (which supports the pharyngeal arch) develops into the lower jaw as well as the ear bones (malleus and the stapes). The gill slits will then close, leaving just one open for the development of the ear opening
• By the fourth week a clear tail is seen in the human embryo. It recedes after a few weeks and these tissues form what is commonly known as the tailbone (coccyx).
• Around the fifth month of gestation the embryo develops lanugo, a fine, downy hair, which covers its entire body. It provides some insulation, as the child has little in the way of fat reserves. This hair is usually lost by birth, though is often seen on premature infants.
6 comments:
I'm so excited by your baby, darling!!
When is he/she due?
xox
I might add that while my two boys did not come bearing a tail, they do still have a fine pair of horns...
hmmm...
Congratulations on your developing little one, and I'm glad you're back to blogging, too! Responding to the comment you left on my blog, if you want to host The Boneyard sometime in October or the 1st week of November, that would be great. Otherwise keep the good stuff coming!
Congratulations and I hope you have all the joy that can go with ushering in a new life.
I've spent the evening writing a snarky letter to the local rag in response to an creationist who has the ignorance or dishonesty to claim there's no evidence for evolution and spefically no transitional fossils, so it's nice to see something happy, even if it is for someone I only know as an occasional blog reader. And, to tie the two together, you're doing what make us different from Paley's watch.
Thanks for your comments, I'm happy to be back blogging and looking forward to our new arrival: a baby boy at Christmas:)
Congrats on the baby news! I've just found your blog and really enjoying it. :-)
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