Mr. Pollard was one of my favorite teachers. He taught us biology in grade 10 and again in my graduating year. The latter course was called “Diversity of Life”. Mr. Pollard shared with us his appreciation for the bewildering variety of living species. Yet, they weren’t so bewildering once he introduced us to the tree of life. He conveyed all this to us with passion and curiosity tempered with humor.
He showed us the similarities and the differences in the diverse categories of living creatures. Best of all, he showed us how every species on earth shares the same foundation of DNA. We learned the standard model of genetics at the time. He showed us that we all have one common ancestor; a single cell. He explained to us why evolution is a proven scientific fact. Biologists have discovered a great deal more since those days in the 70s, but I will always be grateful for the foundation in biology and the appreciation of nature that I owe to Mr. Pollard.
There was one question that every class asked Mr. Pollard that he knew neither he nor any other biologist could answer. That question was “how did life begin in the first place?” When and where did that first living cell come from, and how did it form? Mr. Pollard admitted that it seemed as if someone took a bunch of spare parts, threw them into a bag, shook it, and a car drove out.
It’s been forty years since those classes, and the answer is still a mystery. To deal with the question, we have to go back to how the earth itself formed. That happened about 4.6 billion years ago. The standard model is called the nebular hypothesis.
It says that there was a cloud of molecules floating in space. That cloud collapsed. Most of the cloud’s mass gathered in the center. That’s where the sun came from. The rest spread out into a disc shape. All the planets, including the earth, formed out of that disc through a process called accretion. Little by little, the dust in that disc clumped together to form them. The moons, asteroids, meteorites and comets also formed that way. We can tell that all this happened 4.6 billion years ago by radiometric analysis of the meteorites that have landed here on earth.
Our question, though, is when life formed on our planet once the earth took shape. We’ve all seen the craters on the moon. They bear witness that the solar system used to be a more violent place. Comets, asteroids and even planetesimals like the moon were constantly colliding with the earth. This heated the earth’s surface so much that all the rocks melted. Nothing could have lived in that environment. Anything organic would have been sterilized before it could even form. Complicating the situation further, when rocks get that hot, it resets their radioactive ages, making them impossible to date.
The reason for all this bombardment has to do with the outer planets in our solar system; the gas giants, Jupiter, Saturn, Neptune and Uranus. These planets started out much closer together than they are now. Soon after they formed, they started to shift out to their present positions. This stirred up all kinds of debris that ended up crashing into the earth, the moon and all the inner planets.
The conventional wisdom has been that this took place about 3.8 billion years ago. This was based on samples collected by the Apollo astronauts during the moon landing. Scientists now realize that these lunar rocks were all part of one big impact, so their age doesn’t answer our question after all. The meteorites that land here on earth offer a much better sample for our purposes.
In a paper published last week, a team of researchers led by geologist Stephen Mojzsisof the University of Colorado Boulder has changed that. Using the meteorite data and computer modeling, they have shown that the giant planet migration happened much earlier than thought. They find that it took place 4.48 billion years ago. This is a major shift in our understanding of our planet’s natural history.
This means that life on earth had much more time to form than scientists thought. The earliest specimens in the fossil record are 3.5 billion years old. These findings tell us that the earth could have supported life as much as 4.4 billion years ago. Nothing that has happened since then would have sterilized the planet again.
This clears up something that has puzzled scientists for decades. It seemed as though life had emerged abruptly in a short space of time. These findings add almost a billion years to the time window in which life arose. That makes much more sense.
Mr. Pollard’s question still isn’t answered. Yet, these findings uphold his wonder at the diversity of life and the natural order forty years on. Our universe and our biosphere have a mysterious self organizing property. From that molecular nebula, our sun and its planets took shape. Our neighboring planets somehow organized themselves into the stable orbits we see today. The planetary bombardment ended right after the earth took shape. This gave our planet the time it needed to generate primordial cells from the star stuff dissolved in its oceans.
Scientists around the world are working on this question as well as the subject of abiogenesis, the origin of life from non-living matter. They are confident that they will have answers for Mr. Pollard and the rest of us before long. Then, we can fill in more gaps in the story we wish we could tell about where we came from.
There is always more to learn if we dare to know.