Evolution Biol 4802

Lecture 7, Chapter 5


Topics for today

History of life on earth

    1. Archaean
    2. Proterozoic
    3. Paleozoic – Cambrian explosion


Why is there little evidence of early life?

  • Earth still volatile
  • Erosion, volcanoes, plate tectonics obliterated evidence
  • Ongoing bombardment by meteors
  • Complex aggregations of molecules that left no fossil record

Early organisms left chemical traces

  • 13C:12C ratio in atmosphere is ~1:99
  • Living organisms prefer 12C because its more reactive
  • Graphite deposits in 3.7 byo Greenland rock have isotopic signature of life

Experimental conditions of early earth produced basic building blocks of life

  • Purines, pyrimidines, and amino acids

Other experiments have synthesized

  • Artificial cells
  • Artificial cell membranes
  • Chemical reactions that could have built cellular material from nonliving sources

The chicken or the egg? Protein or DNA?

  • Proteins can do all kinds of complicated tasks except replicate themselves
  • DNA can storing and transmitting genetic information but cannot do any biological work
  • 1980’s small enzymes made of RNA, ribozymes, were discovered that could break nucleic acid bonds

RNA can do it all! (It’s not just the poor cousin of DNA!)

“RNA world” hypothesis

  • Ribozymes evolved into:



  • short sequence of RNA

o       Timeline from Big Bang to DNA based life

Fig. 5.2 (old and new)

For >1/2 of life’s history only prokaryotes were present

  • Concept single ancestor too simple

Fig. 5.4 (old and new)

Deep roots of the tree are difficult to discern

  • Different genes tell a different story of relationship
  • Early prokaryotes swapped genes (horizontal gene transfer)

Prokaryotes continue to swap genes today

  • Pathogenic - 1,387 different genes
  • Benign - 528 different genes

Early life

  • 3.5 byo stromatolite fossils have same structure as those formed today by cyanobacteria


•         Origin of life

•         Diversification of bacteria

•         Increased oxygen in atmosphere

•         Aerobic respiration


•         Earliest Eukaryotes

•         Origin of Eukaryotic kingdoms

•         Multicellularity

Fig. 5.6 (5.5 in older version of the book)

Origin of the Eukaryotes

Advantages of endosymbiosis:

o       Mitochonridia provided aerobic metabolism

o       Choloroplasts provided autotrophy

Molecular phylogeny shakes up our understanding of Eukaryotic kingdoms

Fig. 5.5 (does not appear in the older version of the book)


•         Cambrian explosion

•         First large morphologically complex animals

•         Prior to the invasion of land

•         10-25 my period (geological blink of an eye)

•         Almost all modern phyla and classes of marine animals and many now-extinct lineages appear

Fig. 5.7(old and new)

How do we know about the Cambrian explosion?

o       505 mya an enormous mud slide off the coast of Canada smothered teeming marine life below

o       Successive mudslides created Burgess Shale

o       Uplifted 2 miles into the Canadian Rockies

o       Discovered in 1909 but not understood until 1970s

Fossils of the Burgess Shale

o       Fundamental body plans

o       Some internal organs are preserved

Fig. 5.10 (old and new)

Present during the Cambrian

o       Earlier fossils in Chengjiang, China

o       Later fossils in Burgess shale, British Columbia, Canada

Fig. 5.11 (old and new)


o       Higher oxygen concentration allowed

·        Higher metabolic rates

·        Larger size possible

·        Made tissue development possible

·        Efficient metabolism powered movement

How could we tell whether it was really explosive or just had a long fuse?

  • Molecular clock
  • Estimated rate of hemoglobin evolution among vertebrate groups with known fossil ages
  • Compared hemoglobin of vertebrates and invertebrates and back calculated the time when they must have diverged
  • Earliest branching occurred 1000 my prior to fossil record evidence
  • Progenitors probably small and/or lacked skeletons