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Day 28 (July 23, 2011): Isotopes

July 23, 2011

a woman wearing a cowboy hat, standing in a dry gully, is caught in mid-swing with a large geologic hammer over her head as she digs into a small ledge of bare brown soil.
Naomi swings into action: She wields her geological pick in order to reach the level of carbonate nodules in an ancient soil, which she studies using isotope chemistry.
Evolution doesn’t happen overnight. Instead, changes in the environment usually take thousands of years to be represented by evolution in the morphology, or biological form, of organisms. Yet bones and teeth (and other bodily substances, like shells for organisms that have them) can also record information about their immediate surroundings. These bodily tissues grow within the organism’s lifetime, and they’re built up from the foods organisms eat and the water they take in.

Maybe you’ve never thought about it this way but… food and water are chemicals – and so chemistry offers a way of looking at how an animal’s bodily tissues, such as teeth, or substances made by plants, such as nodules that grow around the roots of plants, reflect the immediate environment of those organisms.

This is the kind of work that Naomi’s does.  Her area, the study of isotope geochemistry, can shed a bright light on the ancient environments inhabited by the animals and plants that lived at the time of early ancestors at Olorgesailie and many other sites around the world. Naomi has kindly guided a little lesson in this area of research.  Please flex your mental muscles now!

First, what is an isotope? Isotopes are atoms of the same chemical element that have different masses. All elements are defined by their number of protons while their mass is equal to the number of protons plus neutrons. Generally, the number of protons and neutrons are equal. For example, Carbon has six protons. In its most abundant form, 12C, Carbon also has six neutrons (6 + 6 = 12). However, there are other forms of Carbon. 13C for example, has 7 neutrons. Because 13C has seven neutrons, 13C is slightly heavier than 12C.

Why do we care? Well, in nature, certain natural processes are picky, that is, they prefer some molecules that contain one of the isotopes over another.  For example, when a pool of water evaporates, the water molecules with lighter isotopes evaporate first, leaving behind water composed of the heavier isotopes. This pickiness, or process by which nature sorts some isotopes over others, is called fractionation. This pickiness is expressed in plant photosynthesis—the process by which plants obtain energy from the sun.

 13C: 12C12in plants < 13C: 12C12 in atmospheric CO2 

Note: Here, we are discussing stable isotopes, that is, isotopes that do not decay. Stable isotopes stay the same and do not change over time. You may have heard of Carbon-14, or C14.  C14 is an unstable, radioactive isotope, which means that it decays over time and is therefore useful in dating organic materials.

So, how can we use isotope ratios to paint an environmental story from the past? First, we have to use what we know.  Let’s continue with the example of plants.  We know from botany that different types of plants use different pathways of photosynthesis, known as C3 and C4 (not to be confused with carbon isotopes!)  In the tropics and at low elevations, grasses usually use the C4 photosynthetic pathway, while trees and shrubs use the C3 pathway.

These two pathways fractionate carbon isotopes differently. We can think of this as the pathways having different degrees of pickiness.  The C3 pathway is pickier than the C4 pathway, that is, it uses the lighter form of Carbon (12C) even more so than the C4 pathway. Therefore, trees or shrubs that use the C3 pathway will have a smaller ratio of C13: C12 than grasses, which are C4 plants.

How does this relate to paleoanthropology? Well, we are what we eat.  When an animal eats plants, the isotopic signatures of those plants become embodied in the animal’s tissues, including hard tissue like bone and teeth.  Thus, we can sample a fossil tooth, measure the 13C/12C ratio of that tooth, determine whether the animal was eating C3 plants or C4 plants, and thus determine if the animal was eating grasses or trees/shrubs! Wow!

Animals which eat mostly grasses are called grazers, while animals that eat mostly leaves from trees or shrubs are called browsers.  Animals that fall in the middle of the spectrum are called mixed-feeders. Here’s a handy guide in thinking about this:

Animal:           Browsers <----------- mixed-feeders ------------> Grazers

Diet:                trees/shrubs                                                                       grasses

Pathway:              C3                                                                                        C4

Examples of browsers include black rhino, giraffes, and kudu, while grazers include white rhino, buffalo, zebra, and oryx. Mixed-feeders include impala and hippos.

What animals do today is not necessarily what they did in the past. Perhaps sudden environmental fluctuations made a browser shift and start eating more grasses. While such a dietary shift would not show up immediately in the morphology of an animals’ teeth, it would be present in the isotopes of their teeth.

A woman in a brown hat, green checked shirt and white pants crouches close to the barren rocky ground as she collects carbonate samples using a small geologic hamme
Naomi samples carbonates at the base of one of last year’s excavations.
Thus, Naomi’s work on the chemistry of fossil teeth can tell us what type of vegetation dominated the landscape at Olorgesailie at any one time.  The vegetation can be studied even more directly by using stable isotopes to analyze carbonates formed by plants in old soil layers. So both teeth and soils offer key pieces of information in understanding why early humans were attracted to this region and the kinds of environmental changes they had to adjust to.