2.6: First Cells

How do you make complex cells?

You start with simple ones. The first cells were most likely primitive prokaryotic-like cells, even more simplistic than these E. coli bacteria. The first cells were probably no more than organic compounds, such as a simplistic RNA, surrounded by a membrane. Was it a phospholipid bilayer membrane? Probably not — it was likely a simplistic membrane able to separate the inside from the outside. Over time, as other organic compounds such as DNA and proteins developed, cells also evolved into more complex structures. Once a cell was able to be stable, reproduce itself, and pass its genetic information to the next generation, then there was life.

The First Cells

What was needed for the first cell? Some sort of membrane surrounding organic molecules? Probably.

How organic molecules such as RNA developed into cells is not known for certain. Scientists speculate that lipid membranes grew around the organic molecules. The membranes prevented the molecules from reacting with other molecules, so they did not form new compounds. In this way, the organic molecules persisted, and the first cells may have formed.Figure below shows a model of the hypothetical first cell. Were these first cells the first living organisms? Were they able to live and reproduce while passing their genetic information to the next generation? If so, then yes, these first cells could be considered the first living organisms.

Hypothetical First Cell. The earliest cells may have consisted of little more than RNA inside a lipid membrane.

LUCA

No doubt there were many early cells of this type. However, scientists think that only one early cell (or group of cells) eventually gave rise to all subsequent life on Earth. That one cell is called the Last Universal Common Ancestor (LUCA). It probably existed around 3.5 billion years ago. LUCA was one of the earliest prokaryotic cells. It would have lacked a nucleus and other membrane-bound organelles. To learn more about LUCA and universal common descent, you can watch the video at the following link: http://www.youtube.com/watch?v=G0UGpcea8Zg.

Photosynthesis and Cellular Respiration

The earliest cells were probably heterotrophs. Most likely they got their energy from other molecules in the organic “soup.” However, by about 3 billion years ago, a new way of obtaining energy evolved. This new way was photosynthesis. Through photosynthesis, organisms could use sunlight to make food from carbon dioxide and water. These organisms were the first autotrophs. They provided food for themselves and for other organisms that began to consume them.

After photosynthesis evolved, oxygen started to accumulate in the atmosphere. This has been dubbed the “oxygen catastrophe.” Why? Oxygen was toxic to most early cells because they had evolved in its absence. As a result, many of them died out. The few that survived evolved a new way to take advantage of the oxygen. This second major innovation wascellular respiration. It allowed cells to use oxygen to obtain more energy from organic molecules.

Summary

• The first cells consisted of little more than an organic molecule such as RNA inside a lipid membrane.
• One cell (or group of cells), called the last universal common ancestor (LUCA), gave rise to all subsequent life on Earth.
• Photosynthesis evolved by 3 billion years ago and released oxygen into the atmosphere.
• Cellular respiration evolved after that to make use of the oxygen.

Explore More

Use the time slider in this resource to answer the questions that follow.

• Evolution at http://johnkyrk.com/evolution.swf.

1. When did the first cells appear? How did these cells develop?
2. Describe the first cells.
3. What was the role of RNA in the first cells?
4. When did photosynthesis begin? Discuss the significance of the evolution of photosynthesis.

Review

1. What was LUCA? What were its characteristics?
2. Which evolved first, autotrophs or heterotrophs? Why?
3. Why could cellular respiration evolve only after photosynthesis had evolved?