Evidence for Common Ancestry (College Board AP Biology)

Membrane Bound Organelles, Linear Chromosomes and Introns

• Membrane bound organelles show that there is common ancestry in all eukaryotic organisms
• Eukaryotic cells contain linear DNA (that has distinct ends) as opposed to the circular loops of DNA found in prokaryotes
• Eukaryotic genomes contain introns, sections of noncoding DNA
• DNA found in the nucleus, mitochondria and chloroplasts of cells can be sequenced and used to show evolutionary relationships between species
• The differences between the nucleotide sequences (DNA) of different species can provide a lot of information:
  • The more similar the sequence, the more closely related the species are
  • Two groups of organisms with very similar DNA will have separated into separate species more recently than two groups with less similarity in their DNA sequences

• DNA sequence analysis and comparison can also be used to create family trees that show the evolutionary relationships between species

Evolutionary Tree of Primates Based on DNA Sequence Analysis

Example of a family tree showing the relationship between primate species

DNA Analysis and Comparison

• DNA is extracted from the nuclei of cells taken from an organism
  • DNA can be extracted from blood or skin samples from living organisms or in some cases, from fossils

• The extracted DNA is processed, analyzed and the base sequence is obtained
• The base sequence is compared to that of other organisms to determine evolutionary relationships
  • The more similarities there are in the DNA base sequence, the more closely related (in that the less distant the species separation) members of different species are

• In 2005, the chimpanzee genome was sequenced, and when compared to the human genome it was discovered that humans and chimpanzees share almost 99% of their DNA sequences, making them our closest living relatives
  • In 2012, the sequencing of the bonobo genome also revealed that humans and bonobos also share 98% of their genome (with slight differences to the differences seen in chimpanzees)

DNA Base sequence Comparison Diagram

The DNA base sequences of two closely related species being compared - Species X is the ancestor of Species Y

Mitochondrial DNA

• When analyzing DNA from the mitochondria is important to remember that:
  • A zygote only contains the mitochondria of the egg and none from the sperm so only maternal mitochondrial DNA is present in a zygote
  • There is no crossing over that occurs in mtDNA so the base sequence can only change by mutation

• The lack of crossing over in mtDNA has allowed scientists to research the origins of species, genetic drift and migration events
• It has even been possible to estimate how long ago the first human lived and where
  • Mitochondrial Eve is thought to have lived in Africa ~200,000 years ago
  • The estimation of this date relies on the molecular clock theory which assumes there is a constant rate of mutation over time
  • The greater the number of differences there are between nucleotide sequences, the longer ago the common ancestor of both species existed
  • The molecular clock is calibrated by using fossils and carbon dating
  • A fossil of a known species is carbon dated to estimate how long ago that organism lived
  • This mtDNA of this species is then used as a baseline for comparison with the mtDNA of other species