Fossil Friday: Thar She Blows; Whale evolution.

Thar She Blows: Evolution of whales!

by Rich Feldenberg

Welcome back to Fossil Friday.  The evolution of the cetacean group (marine mammals like whales and dolphins) is one of the coolest and most beautiful demonstrations of a clear link of fossil evidence from primitive forms to modern forms with many transitional fossils present.  

Whales, are of course, mammals and descended from air breathing land vertebrates.  All tetrapods descended from lobe finned fish (see my Tiktaalik post).  From there they diversified into amphibians, reptiles, mammals, and birds.  Some of the groups of reptiles (like the great marine reptiles during the age of the dinosaurs), and mammals (like the whales) returned to the sea many millions of years later.  Based on molecular genetics studies, the closest living land mammals to the whales is the hippopotamus.  

Cute little Pakicetus was one of the earliest known proto-whales.  These hoofed footed mammals were alive about 50 million years ago.  Based on bone structure of the skull around the auditory region, they fit into the cetacean group, but were not thought to be good swimmers.  Good swimmers in the family would come later!

It is thought that changes in the regulation of genes such as Sonic Hedge Hog (Shh) and Tbx4 may have been important in the loss of the hind limbs in the cetaceans.  By affecting when and how genes are expressed, major changes in structure can be made due to relatively small genetic changes. It is also pretty amazing to see the embryology of modern whales also betrays their ancestry. For example, in the whale fetus the nostrils start out in the usual position for a mammal, but as the maxillary bones grow to huge proportions this forces the nasal bones to the top of the skull. This type of evolutionary effect is called allometry and refers to a change in body parts due to changing the growth rate of different parts in relation to one another.

Over time the cetaceans evolved their characteristic echolocation apparatus, as well as, the development the blow hole from nostrils that were originally forward on the face.  Today, cetaceans are beautifully adapted for life in the oceans.

References and a cool video to watch:

1. Whale evolution Wikipedia:

https://en.wikipedia.org/wiki/Evolution_of_cetaceans

2. Animated video of whale evolution. This is pretty cool, check it out.

http://ocean.si.edu/ocean-videos/evolution-whales-animation

3. Sonic hedge hog:  Wikipedia

https://en.wikipedia.org/wiki/Sonic_hedgehog

4. Tbx4:   http://dev.biologists.org/content/130/12/e1205.full

Atomic Tuesday: The Magic Numbers of Nuclear Physics

Atomic Tuesday:  The Magic Numbers of Nuclear Physics.

by Rich Feldenberg

Similar to the way electrons reside in atomic orbitals around the atom, the protons and neutrons that make the atomic nucleus are also organized into orbitals or shells.  The Nuclear Shell Model addresses the structure and energy levels associated with these nuclear shells.  No two protons or neutrons (nucleons) can be found in the same shell if they contain the same quantum state (again very similar to the quantum rules followed by electrons going into atom orbitals- Pauli exclusion principle). 

Each energy shell can hold up to a certain "magic number" of protons or neutrons, but all nucleons within the shell must be of different quantum states.  If all the quantum states for that shell are already taken, then they will go to the next available shell.  The magic numbers are 2, 8, 20, 28, 50, 82, and 126 - indicating the number of nucleons possible in each of the shells.  If an atom happens to contain a magic number or protons or neutrons it is found to be very stable, and these also correspond to atoms that are the most prevalent in the environment.   An example would be element 10 (2 for the first shell plus 8 for the second shell).  Element 10 is neon which is a stable nucleus.

If an element has both a magic number of protons and a magic number of neutrons it is "doubly magic", and has a tightly bound nucleus.  An example would be Lead-208, which has 82 protons and 126 neutrons.  Heavy elements like lead (Pb) usually have more neutrons than protons since the electrostatic repulsion of the protons needs to be balanced out by more neutrons which provide the strong nuclear force to keep the nucleons bound together.


References:
1. The Nuclear Shell Model; University of Nebraska

2. The Nuclear Shell Model: University of California

3. The Nuclear Shell Model - Wikipedia