Солнечная система и ее тайны

Планеты Созвездия НЛО

The Timescale of Life's Rise

Early in a planet's development the solar system is still being cleared of debris. By "being cleared of debris," we mean that massive asteroids are attracted to the planet, resulting in impacts that are both frequent and severe. Some impacts can be so serious as to remake the surface of the entire planet, melting it back into lava. Life cannot survive under such circumstances, and it is possible that life arose several times early in Earth's history and was then destroyed by such an impact.

After the last period of heavy bombardment, life would have had more opportunity to spread and diversify, but would still be vulnerable to local extinction events. The impact of a lesser asteroid or eruption of a volcano could wipe out the first cells easily, forcing life to start over. The question is, how quickly could life spread across the planet so as to be immune to such localized impacts? Would it take thousands of years? Millions of years?

Let's make an estimate. We will imagine that the first cells do not move - they just sit in place and replicate, pushing each other away as they grow. Let us further imagine that they do not divide very rapidly. Some modern bacteria can divide every half-hour or even faster; let's imagine that these cells divide only once every few days. In fact, let's assume that some of them don't survive, and their total population doubles only once per week. If these cells can find the nutrients they need to survive, how long would it take them to spread across an Earth-sized planet?

The smallest of modern bacterial cells are about 2?10?7 meters in radius. This gives them an area of ?r2=3.14?(2?10?7)2=1.25?10?13 square meters. Earth's surface area is 5?1014 square meters. To cover the entire Earth with a single layer of bacteria, you would need 4?1027 bacteria.

If we start with just one cell on "week zero", let's look at how many cells we would have every week:

Continuing the table will become very tedious. To make things faster, we can describe this doubling with an equation:

where N is the total number of bacteria, and w is the number of weeks that have passed. To find out how many weeks it would take to cover the whole earth with bacteria, we plug in the number we want for N and solve for w.

Solving this problem involves a logarithm:

Plugging this into a calculator gives us a value of about 91.7. Wait - 92 weeks? That seems too small! Let's check out our table at that time. When do we get 4?1027 bacteria?

Sure enough, it would take our slow-growing, non-moving microbes less than 92 weeks - under two years! - to cover the surface of the Earth.

Once life evolves to the point where it can replicate, it can spread very quickly. Life that is fragile and easily destroyed in one location becomes much more robust when it can survive in another location. After the last of the major planet-sterilizing asteroid impacts, life will quickly become so spread out that no single impact event can destroy it all.

Of course, this life will need to be well-protected by its planet, and that will be the subject of the Life on Super-Earths unit.

Footnote 1: This estimate makes several assumptions about the availability of nutrients, the doubling time for early life, and the size of the planet involved. We have tried to make it an overestimate, but it is still a naive approach to the question. Factors such as inhospitable barriers to travel, overcrowding, a lack of widespread nutrients, and depletion of the nutrients that do exist would extend the time. On the other hand, factors such as winds and ocean currents can spread bacteria across great distances.

Footnote 2: Our best estimate for the number of bacterial cells currently on Earth is about 1030, which, if put in a single layer, would cover Earth's surface more than thirty times over.

Footnote 3: All bacteria have some environments in which they thrive, and some environments that are hostile to them. It may take some time for evolution to adapt bacteria to survive in new conditions. This will slow down the spread of life. However, as you may remember from the extremophile discussion, life can adapt to an absolutely astonishing number of different conditions.

The Bacterial World

Below is a segment of an interview with Harvard professor Andy Knoll, talking about how our world is primarily a home to bacteria. You can see the full interview later in the course.

Солнечная система и ее тайны