D.3a: Primate Evolution

23/09/2013 § Leave a comment

I’m so sorry, I’m so so so sorry, I know this is late (shh) but let’s talk about hominids and primates and skulls. Primates belong to the order of mammals and include apes, monkeys, tarsiers (which can be found in Bohol, Philippines, and they are warm and harmless and utterly adorable) and lemurs. Humans are also classified as primates because we share the characteristics of the other members in the order. These include:

  1. grasping limbs (four fingers and an opposable thumb, thank you Ross Geller)
  2. mobile arms (that can move in three planes)
  3. forward facing (or stereoscopic) vision
  4. skulls made for remaining upright

Some important observed evolutionary trends to note because I’m trying to make the writing part of this blog faster than usual:

  • species in the family Hominidae show increasing adaptation of bipedalism
  • ^ the above also show an increasing brain size relative to body size (their heads got bigger)


Page 318, dating fossils using radioisotopes

1. The half-life of 40K is 1250 million years. Calculate the percentage of atoms of 40K that remain in a sample

a) 2500 million years after it was formed. = 25%

b) 5000 million years after it was formed. = 6.25%

2. Estimate, using the decay curve, the percentage of 40K atoms that remain 1875 million years after the formation of a sample. Approximately 35%.

3. 40K decays into 40Ar. Explain, with reference to the decay curve, the reason why 40Ar/40K ratios cannot be used to obtain accurate ages for specimens that are less than 10,000 years old. Okay, let’s look at it this way. Just 1 half life is already 1250 MILLION years long. If you consider this, then the closer you get to 0 half lives, the closer you’ll get to 10,000 years, right? But that number would be, like, 0.000005 half lives, (actually it’s exactly 0.0000008 half lives) but the ratio between 40Ar and 40K is like 1 to 0, which isn’t helpful for scientists. 10,000 years isn’t enough for the potassium to decay into argon in sufficient amounts to create a ratio that can actually be studied.


a) State the radioisotope that can be used to give accurate ages for specimens less than 10,000 years old. Carbon-14

b) Explain why this radioisotope is useful for this purpose. The half life of 14C is only 5730 years and is useful for dating things that are younger than 10,000 years.

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