During the Reign of the Prokaryotes, the continental plates of today, estimated to be about 20 miles thick, were pretty much in place. The location and configuration of those plates can make a huge impact on environment. Earth today has these distinctly separated land masses: the Americas, with one fairly narrow connection from North to South; Africa and Eurasia, again with reasonably narrow connection from Eurasia to Africa. Australia and the South Pole are separate.
Over time, more complex life forms continued to develop, BUT they had to live through some truly dramatic changes on earth. Here are just a few examples.
The land masses move – very slowly, but they move. When one drifts into another one, serious damage can be done. Some land is pushed down; some land is pushed up, making mountains. This is how all of today’s mountains were formed.
At times, all the land masses are connected. One big continent. A supercontinent! One big ocean. One big continent. When a supercontinent is formed, different currents disappeared, which also caused prevailing winds to change. When ocean currents change, rapid cooling or rapid warm up can be triggered.
If all land is massed at one location, the land near the center is no longer impacted by ocean temperature. The ocean is too far away. So the center of this huge land mass can display huge variations in temperature and rainfall.
When a supercontinent begins to break up, the oceans can once again flow between them. New ocean currents form. Sometimes they make the land around them warmer; sometimes colder. Life trying to survive must change enough to survive.
At times, huge land masses would drift over the South Pole. Huge glaciers build up. Impact: those glacier use up a lot of water from oceans. Water levels fall. Continent gets colder. Glaciers form that are 2/3 of a mile thick. When most of the planet freezes, science calls it a Snowball Earth. These have happened more than once. When this happens, habitats disappear; life forms at that time struggle to survive.
Back to first life, the little bacteria. When they had appeared, no competition existed – they could reproduce as fast as they wanted to. And they did!
How? To start with, those first prokaryote cells had a membrane with its DNA (genetic structure) floating around in the center. One cell can reproduce without any outside help. A cell made a copy of its genetic structure, splits in two, and eureka, two cells for the price of one, each with the original DNA. Acting alone, they can reproduce quickly — some as fast as every fifteen minutes! Compare that to your mother’s waiting time of nine months. No wonder they spread from their birthplace all around the world.
Those bacteria, remember, ruled the earth for over a billion years, and during that time mutations led to a whole variety of bacteria species. Instead of swimming around independently, they merged together. Green algae, still all over the earth, is just one of those clusters of bacteria.
So how and why did new organisms appear? Near the end of the Reign of the Prokaryotes, two of those clusters – each cluster representing a different species – joined together. Why? Well, it might have been due to a suddenly tough environment – maybe a Snowball Earth. Not sure. Did one want to eat the other for dinner? Not sure. Scientists are sure, though, that one cluster sort of dug itself into the other.
So, one species was living inside the other. Remember the definition of life: The need to have food, use those nutrients for energy and growth, and then get rid of unused waste? As independent prokaryotes, they reproduced quickly, dumping their waste. Sounds pretty unpleasant, doesn’t it? Living in each other’s waste?
In any event, it turns out the invader had something the host needed AND the host had something the invader needed. Thus, the merger was, in a sense, a sign of cooperation. Little by little, they became more dependent. Soon, the invader species was there for good. Two species became one. Sounds romantic, eh?
“For good” meant neither one of the original two species was an independent organism, with their own special DNA, like they had been before. Each had sacrificed its independence to create a brand new organism – an organism with its own DNA. And the new organism was much, much more complex than any of the bacteria species. The new organism’s name: eukaryote, pronounced u-car’-e-oat, with emphasis on the car.
Remember when the prokaryotes avoided extinction? That process that saved them was Darwin’s random mutation and natural selection model. However, changing two different species of bacteria into to a new complex membrane with its own DNA is really quite different. Mutation then natural selection does not work here. Science needs another answer.
Here is how science sees this step happening. Those first bacteria cells had their DNA floating around in the membrane. Think of DNA as chief record-keeper. All those DNA records are in a sort of file; each file controls a behavior. Those various files are called chromosomes. Each chromosome was a set of directions for swimming or providing energy or getting rid of waste and so forth.
Careful studies can see pretty solid evidence that the parts of the new, more complex eukaryote came from the structure of the two bacteria species from which they formed. Scientists can see remnants of some of those “files” (chromosomes) that were in the bacteria which are now in the eukaryote.
This important pattern continues: that which is new builds out of that which already exists.
None of those files can function without available energy to do the task. For the more complex cell, nutrients taken in merge with oxygen to provide that energy. This is true of all animals. A very similar cell merger yielded a eukaryote that used carbon dioxide and sunlight to make energy in a different manner. That merger is called plants. So the distinction between plants and animals followed parallel paths, not necessarily at the same place or time. The difference between plants and animals is in how they make energy.
A key file inside the membrane controlled reproduction. The first step in the reproduction of eukaryotes mimicked that of bacteria.
- First the cell made an exact copy of itself.
- Then that cell would split, making two cells exactly alike.
Again: eukaryotes building on what the prokaryotes had already done. But after that, the process becomes much more complex. Science believes these are the steps:
- The DNA “doubled up” as it is today – with two strands (the earlier bacteria had but one strand.) Perhaps the two built off the single-strand DNA from each bacteria.
- Then, by going through a complicated series of steps confirmed by science, two new cells resulted.
- The two cells were called “gametes.” The brand new gamete cell is a sex cell.
- The result of a complicated series of steps was that one gamete represented one gender and the second gamete represented the other gender.
- Only the sex cell part of the doubled-up strands was used for the new DNA used in reproduction.
- Science named the smaller, quicker gamete “male” and the bigger, slower-moving gamete female.
Exactly when this happened – perhaps a billion years after the first single-celled eukaryotes – two genders had appear. The author’s first thought was the lyrics of a song: “Then peace will guide the planets, and love will steer the stars.”
Wow! Two billion years of life had passed without any mention of male and female. Two species of bacteria got together and, after some time, merged into a brand new, more complicated cell. After a while, that more complicated cell organizes such that sex cells appear that define gender for the first time. Getting to this point took perhaps a billion years after the first eukaryote cell – a billion years for that incredibly important gender identification step to appear!
Anyway, the story has a few more steps. On Friday, I’ll explain.
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