Thursday, 16 February 2017

How the Mammalian Ear Evolved!

Monodelphis domestica
A new developmental mechanism for the separation of the mammalian middle ear ossicles from the jaw | Proceedings of the Royal Society B.

Alternative creationist fact:
  1. The Theory of Evolution is in disarray because there is no evidence for it. It will be replaced by Intelligent Design any day now.
  2. There are no transition forms.

These are of course as true as Donald Trump's alternative facts, i.e. not true at all and made up to mislead people because the truth is too embarrassing. This paper shows the real state of evolutionary biology and shows how a clear evolutionary pathway from the reptilian ear to the mammalian ear may have arisen by a simple, single mutation. A team of animal scientists at the University of Illinois at Urbana-Champaign, King’s College London, and the University of Chicago have discovered, hidden in the development of opossums, a plausible evolutionary path that led from the simple ears of reptiles to the more elaborate and sensitive structures of mammals, including humans.

When mammalian middle ear bones develop, they begin as part of the arch of cartilage that makes up the embryonic jaw. In reptiles, these structures remain connected to the jaw as developmental processes gradually convert the cartilage to bone.

Multiple mammalian lineages independently evolved a definitive mammalian middle ear (DMME) through breakdown of Meckel's cartilage (MC). However, the cellular and molecular drivers of this evolutionary transition remain unknown for most mammal groups. Here, we identify such drivers in the living marsupial opossum Monodelphis domestica, whose MC transformation during development anatomically mirrors the evolutionary transformation observed in fossils. Specifically, we link increases in cellular apoptosis and TGF-BR2 signalling to MC breakdown in opossums. We demonstrate that a simple change in TGF-β signalling is sufficient to inhibit MC breakdown during opossum development, indicating that changes in TGF-β signalling might be key during mammalian evolution. Furthermore, the apoptosis that we observe during opossum MC breakdown does not seemingly occur in mouse, consistent with homoplastic DMME evolution in the marsupial and placental lineages.

Cutting through the scientific jargon, what this is saying is that the mammalian ear probably evolved several times but by a very similar mechanism. The reptilian ear only has one small bone through which sound is transmitted from the ear drum to the inner ear where sound vibrations are converted to electrical signals in the auditory nerves. Mammals, by contrast have three small bones, the other two being derived from bones in the jaw which formed part of the mandibular-maxillary joint.

We came at this project through the approach of evolutionary developmental biology (evo-devo), which looks at the development of an organism . . . to help understand its evolutionary history. [We integrated] aspects of paleontology, cellular and molecular biology, developmental biology, and more. We’re looking at the problem from more than one angle, utilizing all of these methods to solve the puzzle.

Daniel Urban, team leader.
During development of the mammalian embryo these two bones are detached from the mandible when cartilage known as Meckel's cartilage breaks down. They are then free to migrate to the middle ear. This arrangement is thought to result in an auditory mechanism far more sensitive to quiet sounds or sound of certain frequencies.

In order to understand how this developed, the team studied the South American marsupial, the gray short-tailed opossum (Monodelphis domestica) and in particular the role of the gene TGF-β. This gene is known to be involved in the inner ear development of mice. They found that when the gene was blocked with a drug, Meckel's cartilage failed to break down and the bones remained attached to the jaw.

It was truly remarkable how well the developmental stages of our extant opossum model organism matched up with the transitional fossils . . . this makes our study organism, the gray short-tailed opossum, a fantastic living model to aid in the understanding of development of long extinct taxa. By using this modern analogue, we can learn so much more about these earlier species and the origins of mammals.

Daniel Urban, team leader.
What this shows is that in marsupials, the evolutionary mechanism could have been as simple as a single mutation in a single gene. The adaptive driver than may have driven the spread of this gene through the population could well have been something that probably led to several mammalian features - a nocturnal, insectivorous existence. The early mammals probably became nocturnal because dinosaurs dominated the diurnal environment but were not so active in the dark. There would have been a significant advantage to have sensitive ears in the dark when hunting for insects.

So, contrary to notions creationists like to comfort themselves with, the Theory of Evolution offers a ready explanation which explains the fossil evidence and involves an easily understood transitional mechanism.

Perhaps a creationist would like to offer an explanation for a mammalian embryological development that begins to grow a reptilian jaw then kills off some of the cells to free up a couple of bones that are then re-used for a function unrelated to their original one. Why would an intelligent designer not simply design an efficient middle ear complete with the normal compliment of bones in the first place?

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