Chemistry to the rescue!!!

Last Wednesday I decided to catch some Southern California sun during lunch time. Sitting on the concrete perimeter of the reflection pool outside Leavey Library at USC, I was enjoying my chicken pesto sandwich when I was approached by a petite Korean girl. She was in her early twenties wearing a denim mini skirt with a pink tank top.

As she sat next to me, we started talking. It was nice to have company as I was working my way through the sandwich. Imagine my surprise when she asked me if I had found Jesus in my life, after a mere five minutes of meaningless banter!

As a practicing Hindu, it was fun to discuss our religious beliefs, morals and values and our understanding of human life itself. But what struck me the most was the conviction with which she believed that our planet was about 6000 years old; even more, the fact that evolution is a conspiracy designed to undermine Christianity.

Our discussion soon turned into my fierce defense of science as I tackled the question of our planet’s age. I laid out geological measurement of time from thickness of the ice cores in the Antarctic, the slow movement of tectonic plates that have shaped the planet and then moved onto the chemistry used to determine our planet’s age.

I explained that everything is composed of unique materials called elements, the identity of the which is determined by the number of positively charged particles called protons within the nucleus, or center, of the atom, around which the same number of negatively particles called electrons revolve, just like the planets around the sun. Also residing within the nucleus are neutral particles called neutrons.

I continued explaining that each element can exist in multiple forms called isotopes. Some isotopes are stable, others are not. The unstable ones break down to give stable elements at a constant rate in a process called radioactive decay. Using this constant decay rate, the age of rocks, fossils and the earth itself can be predicted. Using the uranium-lead radioactive dating, earth’s age is calculated to be about 4.5 billion years, I concluded.

Having established to my satisfaction, but not necessarily hers, that earth is very old, I moved on perhaps the more controversial topic of evolution. Being a biologist, this was home turf for me. I took her from a prehistoric earth where life first evolved as microbes to the present day, interspersing mass extinctions, identification of missing links and finally our development as a species.

Just as Uranium-lead radioactive decay was used for measure the age of the planet, age of fossils is determined by carbon-dating. Elemental carbon exists in three major forms- C12 with 6 protons and 6 neutrons, C13 with has one extra neutron and C14 with two extra neutrons. While the first two isotopes are stable, C14 is not. It constantly decays such that its quantity is halved every 5780 years.

Measuring the relative abundance of this isotope underlies the radiocarbon dating we use to estimate the age of most fossils upto about 60,000 years back.

She respectfully listened to me. But still believed that our measurements were incorrect. I know she was not convinced, but I was hopeful that our discussion would make her give science a second chance.

As I left for my class, I remembered something from an undergrad biology lecture. Most biological molecules do not survive even a few million years. How did we ever know that life existed 2 billion years ago? I couldn’t believe I had never given a moment’s thought to this.

Seeking an answer, I spoke with Dr. Kenneth Nealson, Wrigley Chair in Environmental Studies and Professor of Earth Sciences and Biological Sciences at the University of Southern California. “We have geology to sink our claws into”, he explained.

“If you had a twelve-pound ball and a thirteen-pound ball to throw over the fence. You started to get tired and pretty soon the twelve-pound ball would be dominant”, he explained.

Similarly as carbon is processed, enzymes prefer to use the lighter C12 to the heavier C13, such that over long periods of time all or most of the carbon in these biological molecules is composed of lower atomic weight carbon. Life prefers low carbon since it requires lesser effort to process it.

If carbon were to be incorporated into molecules by random chance, then the isotopes would be in the ratio of their natural occurrence.

Thus, by comparing the ratio of isotopes in a molecule to its relative abundance, the source of the molecule can be deduced. The same holds true for other molecules significant for biological processes including oxygen, nitrogen and sulphur.

So robust and accepted is this science that this underlies NASA strategy for searching life on Mars. The Mars Science Laboratory onboard the Mars Rover Curiosity, due to the launched on 26^th November 2011 by NASA, has an atmospheric sensor called a tunable laser spectrometer, a device designed to analyze the relative abundance of elemental isotopes in the Martian atmosphere.

If the methane on Mars were an outcome of biological processes, then the results would show skewed abundance of C12 methane, thus pointing to the existence of at least microbial life at some point in history.

There is always more to learn, I told myself as I left Dr. Nealson. I am glad to have been open to talking to a complete stranger.


NOTE: This blog has been submitted to the Nescent Evolution themed Blog Post Contest 2011 for consideration for a travel award to Science Online 2012 conference in Raleigh, North Carolina. More information at http://blogcontest.nescent.org/2011/10/12/win-a-travel-award-for-best-evolution-themed-blog-post/