Season 1 Episode 11: Chapter XI -On the Geological Succession of Organic Beings

Let us now see whether the several facts and laws relating to the geological succession of organic beings accord best with the common view of the immutability of species, or with that of their slow and gradual modification, through variation and natural selection.

In this episode we explore Chapter XI that deals with the fossil record and how patterns in the fossil record fit with Darwin's model of descent with modification. We begin the discussion with Darwin's views on extinction and how this phenomenon is part of his model of evolution. Sarah introduced the idea that extinction occurs at two different rates - normal background rate of extinction where species themselves have a measured lifespan based on how long they exist in the fossil record. Current analysis of the fossil record indicates that many species persist for a period of time and then go extinct. From this  data scientists calculate an average lifespan for a species - That is, how long in the geological record does a species typically persist? Many authors have written about this idea and many of their findings have been summarized in Extinction Rates, a book edited by J. Lawton, and R. May (1995).

Generally species lifespans are varied and biased by the size of the organisms. Small unicellular marine dinoflagellates have species "lifespans" in the 10's million years whereas large multicellular Cenozoic animals had a species 'lifespan" of 1-2 million years. These normal background rates of extinctions in the fossil record are punctuated by five massive extinction events where 75%-96% of all species went extinct. Sarah mentioned the dramatic "soot' line that demarcated the mass extinction event called the K-Pg (formally called the K-T) extinction event that occurred 66 million years ago. This extinction event was correlated with a thin layer of metal iridium that is visually and chemically distinct from rock material layers above and below the line. This layer was now thought to be have been created when a small asteroid impacted the earth near the Yucatan peninsula contributing to massive habitat destruction and climate change. These massive changes in climate are thought to have contributed to the extinction of all non-avian dinosaurs.

image from http://waynesword.palomar.edu/images2/KT1c.jpg

Sarah recommended a book on extinctions by Peter Ward called Under a Green Sky: Global Warming, the Mass Extinctions of the Past, and What They Can Tell Us About Our Future (Harper 2008).

Affinities of past to current organisms

Let us now look to the mutual affinities of extinct and living species. All fall into a few grand classes; and this fact is at once explained on the principle of descent.

We discussed how many of the organisms in the fossil record can be directly related to species we see today - the wide array of extinct elephants themselves are a dramatic example of extinct forms being related to extant forms today.

image from http://img13.deviantart.net/1309/i/2015/120/9/7/elephants_by_fotostomias-d34pdf2.jpg

James spent some time explaining the background story of the archeopteryx fossil that was found in 1861 in Germany just after Darwin published Origin of Species.

Photograph: Jason Edwards/Getty Images

On the photo above notice the fingers on the wing and the long bony tail that extends off of the pelvis region. Museums are now creating displays that highlight the feathered dinosaurs that have been found in the past 10 years.  In 2016 the American Museum of Natural History opened a display that highlighted many new feathered dinosaurs.

http://www.nytimes.com/2016/03/18/arts/design/fine-feathered-fiends-dinosaurs-among-us-at-the-american-museum-of-natural-history.html

 The relationship between organisms we see today to extinct versions in the fossil record is one of the stronger pieces of evidence that supports Darwin's model of descent with modification. In the next episode we will discuss how this relationship between extinct and extant organisms is actually restricted geographically, that is the mammals in the fossil record of South America are similar to the mammals you find in South America today, whereas they are different from those mammals you find, either in the fossil record or living, in Europe, Africa, Australia or Asia.


Ontongeny Recapitulates Phylogeny

Josh introduced the idea that embryos of organisms can reveal their evolutionary past since they often exhibit structures during development that are more fitting to earlier evolutionary forms. It was formally developed by the famous biologist Ernst Haeckel (1834-1919) in which he argued that organisms have developmental stages that represent earlier stages of their evolutionary heritage.

By Romanes, G. J. - Romanes, G. J. (1892). Darwin and After Darwin. Open Court, Chicago., Public Domain, https://commons.wikimedia.org/w/index.php?curid=823180

Although the drawing above has been discredited because Haeckel was said to have taken liberties in scale and overly emphasizing some structures, it is clear that early embryonic stages of many chordates exhibit clear gill arches and a well developed tail that becomes greatly reduced or lost during development. Josh queried us about what type of tail we would most prefer - foxy tail was by the far the best choice.

interlude music is
http://freemusicarchive.org/music/Latch_Swing/demo_2008/Rythme_Gitan