Season 1 Episode 10: Chapter X - On the Imperfection of the Geological Record

...so must the number of intermediate varieties, which have formerly existed, be truly enormous. Why then is not every geological formation and every stratum full of such intermediate links? Geology assuredly does not reveal any such finely-graduated organic chain; and this, perhaps, is the most obvious and serious objection which can be urged against the theory. The explanation lies, as I believe, in the extreme imperfection of the geological record.  [406]

This episode focused on Darwin discussing the state of the fossil record, why he felt transitions may be rare and why he does not think the imperfection in the geological record was fatal to his theory. We discussed three of his major issues with the fossil record and then discussed the age of the earth and the sudden appearance of diversity in the fossil record. A phenomenon often referred to as the Cambrian Explosion.

How to make a fossil? 

One of the challenges of creating a fossil is avoiding the rapid decay and destruction of the organic material by bacteria, fungi and other decomposers.  The majority of the decomposers require oxygen to live so a successful fossilization event requires the remains to settle in an area that is anaerobic (anoxic for aquatic systems) (without oxygen). Anaerobic conditions occur in deeper waters where oxygen is slow to replenish from the surface, or it can occur in wet terrestrial systems like bogs or river banks where wet saturated soils can become anaerobic. We didn't mention in the podcast the amazingly preserved 1,000-2,000 year old humans discovered in peat bogs in Europe. These acidic and anoxic environments deter the growth of most bacteria and fungi.

image from: http://ngm.nationalgeographic.com/2007/09/bog-bodies/clark-photography

This is a picture of a sheep that fell into a ditch that was then frozen over, although it is not technically a bog, it does show the difference between access and lack of access of decomposers to a body.

image from: https://i.imgur.com/S6Hn8i6.jpg

Ultimately the conditions that favor fossilization result in a bias in who gets fossilized which greatly biases and restricts the fossil record. In general you can say that to become a fossil you should be:

• Small and abundant. The more abundant you are in the environment the more likely you could experience the rare opportunity to become fossilized. 
• Live in or near an aquatic environment.
• Have hard body parts than can resist decay. Scales, shells, bones, exoskeletons and woody material often resist decay whereas flesh, hair and soft tissue not so much.
• If you are small, blind, sessile and attached to the sea floor you are more likely to become a fossil or fossil imprint than if you are large, active and run across the Serengeti or swim in the shallow, wave disturbed seas.

Although the fossil record is biased towards small, aquatic organisms there are museums full of large and dramatic fossils that contradict the rules above. So when you consider the improbability of a creature becoming a fossil the fossil record is actually quite impressive!

photo of Sue by James Wagner

Is evolution predictable?

James mentioned that evolution tinkered with different materials or metabolic pathways but ultimately settled on a few systems that are widely utilized across the diversity of life. In particular we discussed how evolution utilized many different chemical mixtures to create a hard support system (endoskeletal, shell, or exoskeleton) with an interesting alternative seen today in the "glass" sponges. Glass sponges utilize silica to create their hard spicules, the support material utilized by sponges to give strength and support to the jumbled array of barely specialized cells that constitute a sponge. Unlike most sponges which create individual spicules which they assemble with the organization that resembles a game of pick up sticks,

spicules by http://www.jeolusa.com/Portals/2/ModuleDNNGridGallery/ncaor_india_sponge-spicules.jpg

the glass sponge silica spicules fuse into a large basket-like structure that remains intact after the sponge dies.

http://oceanexplorer.noaa.gov/explorations/03edge/logs/aug23/media/glass_sponge.html

We discussed the crazy idea that early evolution experienced a fork in the road when it came to building a mineral based hard skeleton - it could have used silica as the material or calcium carbonate/phosphate. Josh mentioned the downside if we had evolved a glass skeleton.

http://www.glasskulls.com/clear-skull/

We also discussed the other major evolutionary fork in the road - green versus purple photosynthetic systems. Here is a beautiful picture showing a mat of purple photosynthetic sulfur bacteria floating among a pad of green algae.

https://sites.google.com/site/botany317/session-2/bacteria/green-purple-bacteria

If we rewound the tape and replayed evolution could we have ended up in a purple world with fragile glass skeletons?



The opening and closing theme to Discovering Darwin is "May" by Jared C. Balogh. http://freemusicarchive.org/music/Balogh/Revitalized_Eyes/MAY 
Copyright: Attribution-NonCommercial-ShareAlike: 
http://creativecommons.org/licenses/by-nc-sa/3.0/   

Interlude music is Little Lilly Swing by Tri-Tachyon which is licensed under a Attribution License.