Return to Teaching Units

ORIGIN of LIFE and
INTRO to the CHEMISTRY OF LIFE

Lots of compelling evidence shows us that evolution nicely explains the diversity of life over time, and most questions about the processes of life. But we are still left with this question: How did the first life come to be?
What are some scientific explanations that attempt to answer that question?

To explore this, we must realize that we are indeed composed of lots of chemicals interacting in many complex ways to produce the phenomenon we call “life.” Every material thing is composed of chemicals, so we assume that there were chemicals in the non-living world of our early Earth before there was life. But what could have happened to some of those chemicals so that they began to come together, to take in chemicals and to obtain energy to grow, move and reproduce?

For this, we need to understand some basic things about chemistry in general, and the chemistry of life in particular. An interesting way to introduce this is to consider what chemicals we’re all made of. Take a look at our Chemistry and Meaning of Life and consider using at least a small part of those ideas to engage your students in seeing life from a chemical perspective.

Then we can look at some of the scenarios explored by scientists that might have produced the first life. A convenient source to explore these ideas briefly can be found at Live Science: 7 Theories on the Origin of Life.   Perhaps you could prepare a PowerPoint series of slides from the 7 images provided there. Most of the current work on this subject is being done with RNA. Certain kinds of RNA can not only make copies of themselves, but also transfer information from generation to generation, just like DNA does now. Also, other RNA molecules can act like enzymes, promoting chemical reactions in living cells. Most enzymes today are made of protein.

Origin of First Life Resources:
The origin of first life is sometimes called “abiogenesis.”
Abiogenesis on Wikipedia
Understanding Evolution: Studying The Origin of Life.
Understanding Evolution: How Did Life Originate? 

One of the features of life is that it obtains and uses energy for its existence, growth and reproduction. The key processes for doing this include photosynthesis, fermentation, and respiration. Photosynthesis is the way certain organisms get energy from the Sun and store it in sugar. Fermentation is a relatively simple process for extracting energy from sugar. Respiration is a more complex process (adding to steps of fermentation, and using oxygen) to extract energy from sugar. An interesting question: how and when did these processes begin? There was little or no oxygen in our atmosphere until a little less than 3 million years ago. Evidence (fossils and chemical clues) tells us that the earliest life appeared around 3.5 million years ago.

This has close ties to how organisms get their energy. Those that simply digest other organisms (like we do) are called “heterotrophs” (“other-feeders”). Those that get their energy directly from the Sun (like plants) are called “autotrophs” (“self-feeders”). Photosynthesis is how most familiar autotrophs do this, but photosynthesis is very complex, and therefore autotrophs were probably not among the earliest/simplest life forms. Consequently, we suspect that the first living thing was probably a heterotroph. For more on this “heterotroph hypothesis” about the earliest living things, go to our Bio-Energetics page, and scroll to the bottom to the “Evolution of Bioenergetics