From one cell to many: Eukaryotes

At first, one cell included both gametes; a cell like that is called hermaphroditic, which means both the two genders still reside in the same membrane. They reproduced with hermaphroditic cells breeding with another. But before long, the two genders separated. One gender produced the egg and the other the produced sperm, just as humans do today. Soon the hermaphroditic cells pretty much disappeared and the single-sex cells dominated. Logic suggests a lot of reasons why that should not have happened:

  • Males compete with one another for the most desirable female with which to mate. Sometimes these competitions get pretty brutal.
  • The process the eukaryote goes through to reproduce is considerably more lengthy in time than is the prokaryotes process. Waste some time, fall behind.
  • And finally, the product of the reproductive process includes males and females. Males cannot reproduce. Thus the system is only half as effective as before.

Despite these, the event happened—and had it not happened, Earth would still have no living things but the single-celled prokaryotes.

The rate of evolutionary progress with the appearance of sexual reproduction increased dramatically. Looking backwards, sexual reproduction was a critical step leading to us. Period. Too many argue, “Well, it had to happen or we would not be here!” That statement assumes that the potential benefits are known by natural selection in advance. But the Darwinian process is randomness; pure, unadulterated randomness. Forecasting the future is not part of the process.

So two prokaryotes species game up their identity to make the eukaryotes but the eukaryotes were not gracious all. Their DNA called for ingesting food. To those new, bigger cells, bacteria were food. In fact, after a while a hole in the greedy eukaryotes membrane appeared, allowing the eukaryotes to swim along ingesting the bacteria. That hole eventually became our mouths. But the gobbling eukaryote was still a single cell.

In time, those special-duty single cells began to work together, sort of like “You make the soil, I’ll get the seeds, Al can plant them, and Louise will pick them.” They attached themselves to one another. But they retained their own DNA. Bigger cells had a competitive advantage. The problem: a single cell had a size limit.

Eventually, in order to get bigger, those the cooperating single-celled gave up their independence, in a one sense, to stay alive. They merged together, and became multicellular. The individual direction-centers were merged into one more complex DNA.

Multicellular means that now a lot of single cells, each with special functions, work together. As life becomes more complicated, so does the DNA at each step of the way. The DNA, remember, is the main library where all the directions are kept. As the organism becomes more complex, progress is maintained in the DNA.

Eukaryotes are the new kid on the block. So what, you say? Plants, fungi and animals will evolve from the new kid on the block. “Animals” include YOU!

timeline

Fossil evidence of this trip from single-celled to multicellular is scarce. Those first organisms were soft; a billion years or so later they do not leave any marks. However, scientists, working in their labs, have done a pretty good job of recreating this last transition. The evidence indicates the trip from single-celled to multicellular was a step-by-step process, a convergence, and not one big dramatic change.

In the absence of fossils, estimates of when this last step happened conflict. Based on evidence available suggest multicellular life appeared about 1.5 billion years ago, or 2.2 billion years after first life began. However, the appearance of two genders will indeed speed things up.

The Beginning of Gender

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.

Next post: Gametes and more

Previous post: Enter Oxygen

Enter Oxygen

On Wednesday, we asked: how does science know all this stuff? The answer: rocks.

Rocks store old information. First, a rock holding such critical information must be found. Once found, then the rock that held that information must be dated. In the rock are elements that have a “nuclear decay” — they shoot off a little energy which basically creates a new element. Time is measured by the amount of the new element in the old rock. The equipment used is pretty sensitive.

Logic also gets involved. If the rock fossils are all single celled, then that rock existed before multicellular life appeared. The single cells are at the lower level; multi-cells one layer up.   In a big, broken stone or a cliff with many layers, clearly each new layer appeared after the one below. By piecing all this information together, geologists can make reasonably accurate estimates for timing of ancient events.

When scientists asked, “What happened?” and “What are the steps that made it happen?” the response usually falls back on Charles Darwin’s natural selection interpretation.   Most people have heard that term “evolution”; unfortunately, however, a large percentage do not really understand the details. All at once, first life was under attack. The next step (which saves the prokaryotes) in the story is an example of now Darwin’s approach works.

Those first little bacteria were flourishing, thriving on an oxygen-free environment. They were having a jolly time covering the earth. But then — the plot thickens — they came under attack.

What attacked them? Oxygen. Photosynthesis arrived near the middle of the Reign of the Prokaryotes. Bacteria capable of photosynthesis were ingesting carbon dioxide and spewing out oxygen. Soon the atmosphere had a lot of oxygen. To the existing prokaryotes, oxygen was poison; they were dying off in droves. If they all die, well, this story ends and there would be no you or me.

Why didn’t then all die? Darwinian interpretation involves a two-step process.

Those bacteria reproduce by splitting in two. After the split, both parts have exactly the same DNA structure. But random accidents happen; sometimes one of the cells does NOT have exactly the same DNA structure. That oddball cell is called a mutation. So start with the first step: random mutation.

The second step is called natural selection. Change is triggered by those reproductive mistakes. The mutation fights to survive – to find food, to reproduce. Most mutations die off; after all, they happened by chance. But along comes a mutation that CAN survive even if oxygen is in the air. That mutation flourishes, reproduces like crazy, and starts dominating. A change has occurred in that bacteria cell’s DNA. Children who come from this somewhat changed DNA are also better-able to handle oxygen. That new, DNA-altered bacteria thrived. The “random” part is the mutation. The “natural selection” here is that nature selected the bacteria that reproduced and thrived most successfully.

Another question: Did this change occur because of just one mutation in one cell? Many believe that other bacteria, under the same strained conditions, could also have created mutations that could thrive on oxygen and be successful. Perhaps there were hundreds of such mutations and, probably, they were not all exactly the same. They could all a little different BUT all can handle living in oxygen. Over time, they will converge to one big happy family, all of whom have a DNA structure altered in a manner allowing them to thrive on oxygen.

A lot of time had passed, but the Reign of the Prokaryote finally came to an end. In their nearly two billion year dominance, evidence shows that all over the Earth, where there was water, some form of bacteria was growing. Sometimes it made its way onto land.

Soon more complex life will join them on Earth. That more complex cell only formed because two groups of bacteria species joined together, merging permanently.

The beginning steps – life beginning, the new life form flourishing and then a more complex form appears – happened in an environment capable of making dramatic changes. Like it or not, each new life form had to be in harmony with the environment around it. Next week, we’ll look at a sort of short description of how these dramatic environmental shifts happen.

Next post: A look at the dramatic environmental shifts that allowed for a new form of life!

Previous post: The Reign of the Prokaryotes

The Reign of the Prokaryotes

Here we pick up from Saturday, with the formation of the planets in our solar system and the continents on our planet, Earth. Most of the junk flying around in space had been gobbled up by the Sun, Jupiter, and the rest of the continents. The Earth finally was freed from a constant bombardment of comets and good-sized asteroids. Damaging asteroid strikes did not disappear; they just became much less frequent.

Volcanoes erupted regularly. Between those violent crashes and the volcanoes, the earth’s boiling hot surface included an atmosphere of methane, steam, hydrogen sulfide (the rotten egg smell), and carbon dioxide, in addition to nitrogen and carbon dioxide. No oxygen, though! The earth finally cooled a little, turning the steam into water. Rain poured down for a long time, filling the oceans.

Around four billion years ago, conditions were cruel. The atmosphere and the oceans were a chemical soup. The oceans were green and acidic, the skies orange with high levels of methane, ammonia and carbon dioxide. A complete list of all the ingredients of both the ocean and the atmosphere are not known for certain, but any living plant or animal of today would immediately die if placed in that environment.

Despite these horrific conditions, life appeared. Out of that fiery, dense Big Bang explosion comes the itty-bitty little particles out of which you, your parents, the ground you walk on, the Moon, the stars – everything! – is made. Little bits of that stuff got together just right and out came the first DNA.

Life begins. In the water. Life’s first DNA. That DNA, though, was NOT simple.

Well-preserved bacteria from the era 3.6 to 3.2 billion years ago was found in Western Australia. General agreement for life’s beginning is 3.7 to 3.8 billion years ago.

What arrived was the earth’s simplest form of life. But what is life?

  • A simple explanation: on one hand are living things, plants and animals; on the other hand, inorganic matter.
  • A more precise definition: Living things take in food, grow, and have wastes; they reproduce; and have DNA.

Those first living organisms were single celled. Biology calls them prokaryotic; “bacteria” is easier to remember.

Here is an introduction to that first life form, the prokaryotes. Each organism was surrounded by a thin membrane. Inside the membrane, nutrients moved around, messages were sent, and a variety of other complex tasks carried out. The instructions for all this action were in the DNA. The DNA directions included how to do those things—move, use food for energy, eliminate waste, and reproduce.

Those first living cells were tiny, tiny, tiny. A piece of paper is about one millimeter thick. Those first cells were 1/1000 of a millimeter! Two examples of prokaryotic organisms today are bacteria and green algae.

Those tiny organisms stored essential genetic information coiled up inside. To create this two-for-one step called reproduction, the cell first had to grow to twice its own size. Then it split into two, creating a matched copy of the original. The process was not really that simple but that is an outline.

Reproduction required no external help. The cell was on its own to grow-split-grow-split- … and on and on. Do not scoff at these prokaryotic cells. They are by far Earth’s most consistently successful organism. The reign of the prokaryotes begins and continues for more than two billion years.

Here is a timeline of the beginning of the story from Big Bang to you.

big bang to you

These little bacteria cells had a little tail; they could move about in the water. Today, they exist in plants and animals as well as in the atmosphere. In your body, they help digest food. They also cause you sickness.

The enquiring mind is saying, “How does science know all this stuff? Do they just make it up?” A complete answer would take a book. Next week, a brief explanation!

Next post: How does science know all this stuff?

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