One question arise how did life begin on Earth, Our earth was formed 4.5 billion years ago and after its formation, many things kept colliding with it such asteroids comets etc. But a few million years after its formation, life was present on our Earth. So how did our earth change from a lifeless planet to one of life?
The Origins of life
How did life begin on Earth (life start on earth) ? This is a difficult question and an important one its implication are also on when astronomers try to find our life on other planets. But let me clarify one thing. We try to find out naturalistic explanations, according to science about the origins about the question of the beginning of things. We do not bring miracles in them, at least from the science perspective.
For example, a few hundred years from today it was presumed science could not answer the question, how did our sun and our earth form.
But today we have established theories as to how our sun and earth were formed about 4.5 billion years ago. As far as the question of the beginning of life is concerned, even a few hundred years ago, that idea was “its no problem”. The reason being most of the people thought that life emerges spontaneously.
Why was this thought?
Actually, an example of it is if meat is left in the open after some hours, or after a day insects and maggots appear in it. People thought that is a good example, that life emerges spontaneously, without any reason. This way, if you think about the beginning of life (life begin on Earth), this must have emerged in the same way.
But after that, some experiments were carried out in the 18th century and then the famous Louis Pasteur (we talk about pasteurised milk or pasteurisation) carried out a fascinating experiment in the 19th century.
He sealed the meat, sterilised (heated) it so that if some bacteria is present it evaporates
And then sealed it, so that nothing from outside can enter. Then it was know that if you sterilise it, and then seal it life does not spontaneously appear in.
Pasteur thought that there are seeds or something like that in the air which brings life or insects in it but if you seal it then they (insects) do not appear. This experiment is amazing by the way, the flask in which Pasteur perform, the experiment is still present in Paris. But because of this another problem arose.
The question was how did life begin on Earth?
We will talk about that but let me give a little classification and qualification also that I am not a biologist. I am an astronomer and my interest is because this question is important when astronomers try to find out life on other planets. So we will talk about this here.
We know that our Earth (life begin on Earth) was formed 4.5 billion years ago. The oldest evidence of life begin on Earth we have is 3.48 billion years old. These are fossilized stromatolites.
This mean life begin on Earth between 3.48 billion and 4.5 billion years. In fact, in the beginning, there was bombardment because comet and asteroid word striking
And a big body named “Theia” also collided with earth in the beginning (life begin on Earth) due to which our moon was formed. The idea is oceans what president on Earth 4.3 billion years ago. This weather time limit of the beginning of life can be shortened more. The guess is that the first life evolved between 3.5 and 4.3 billion years ago.
Before we talk about the origins of life which is also called bio-genesis another big question is how do we define life. This is very difficult question.
I want to recommend a book to you. This is a book by Robert Hazen “Genesis: the scientific quest for life’s origin”. There is muchmy discussion in it about the origins of life or life begin on Earth, and the question about the definition of life. Robert Hazen says in the early 1990s NASA organized a workshop on astrobiology in with this question how to define life was discussed.
Why is NASA interested in this?
If you are trying to find out life on some other planet as same life begin on Earth, you must know how to define life. This is a very difficult question because many people disagree. For example, are viruses life or not because they need a host system. Likewise, if you say, reproduction is necessary for life, then will you count mules in life or not. So you can say this is a challenging sort of question.
Robert Hazen said that in the astrobiology workshop of 1990’s, one definition, on which most people agree was “Life is a self sustaining chemical system capable of undergoing Darwinian evolution”. Ok one thing, but this is a self-sustaining system and the second part is whatever the life form that has the capability to evolve.
Let me say one thing. Many people get irritated by the name, Darwin and evolution. There is nothing in it. Let me a definition of Darwinian evolution is this (in a way), things you have, change, which is called variation part. These are random changes most of the time, and the second part of evolution is selection. Because most of the changes are not beneficial or sometimes even cause harm.
But if there is something beneficial from among many random things, its selection is through this mechanism of evolution. So this combination variation and the second thing selection is a very powerful mechanism to see changes in life.
When we talk about earth, life here began in simple form (life begin on Earth) and if you give it time and principles of Darwinian evolution, then very complex forms of life can also develop.
How did release biology from chemistry on Earth?
Most people agree on some basic characteristics. For example, the idea is a key element for life Carbon, Hydrogen, Oxygen, Nitrogen, Sulphur and Phosphorus played a significant role in the evolution of life and especially this Carbon atom is very necessary, and it is presumed this is central for life.
The reason is carbon atoms can make bonds with other elements with oxygen and hydrogen etc. It is presumed that if we are talking about the origins of life carbon has a central role in it.
The second thing is the presence of water. Search medium, which is better for mixing things, it allows the mixing of different type of molecules. If you have gases (water can be in form of gases is also) The molecules in them are at a distance and moving fast, so they collide at a high-speed. If it is solid you can guess they do not have much space to move so their union is a little difficult.
What is one medium which is necessity, then we talk about the beginning of life. And then energy. Energy comes from the sun also. If you talk about the surface the heat of the Sun provides energy. And now scientist, think a lot of energy can come from the geothermal vents in the Oceans and maybe (some people such as Robert Hazen think) life began on Earth at the hydrothermal vents below the oceans. And the last thing you require is time.
If you have to perform such combination you should have a lot of time and at the beginning of earth there was a lot of time and for several hundred million of years these experiments were taking place on the surface of the Earth. Okay, so now we know raw material which made life and the thing with necessary for life are very common.
But then how did life begin on Earth?
Scientists think that may be, the beginning of life took place in three-steps.
- The first step is the formation of amino acids. These amino acids are crucial for proteins, and nucleic acids, which are important for DNA and RNA.
- The second step is the formation of complex molecule, for example the membrane of the cells or polymers that chain of complex molecules.
- And the third step is formation of such complex molecules which can store information and can replicate or reproduce or make copies of itself, such as a DNA molecule.
First Step of life begin on Earth
Now from these the first step (life begin on earth) of formation of amino acid has been solved to a large extent. A very famous experiment. According to size is the Miller Urey experiment performed in 1952. In this Stanley Miller a graduate student and his advisor Harold Urey (a Nobel laureate) basically simulated the atmosphere at the beginning of our earth. How?
They took a flash and the gases they mixed were some ammonia, some methane, and some hydrogen and heated it (gave energy). And mixed some water in it.
Now just a minute. Some among you will say here there is ammonia, methane hydrogen in the atmosphere.
What about oxygen?
Actually, oxygen was not present in the atmosphere at the beginning, rather oxygen came into the atmosphere because of life. In fact for 2.5 billion years much free oxygen was not present in our atmosphere and the Earth was covered by blue green algae (the early life forms) beneath the oceans.
The oxygen released by them in the reason for oxygen in our atmosphere, which proved beneficial for the life forms, which came afterwards. But there was no oxygen in our atmosphere at the beginning.
The Miller Urey experiment, simulated the early atmosphere provided energy (fir example, sunlight) mixed water and let the elements continue. After this experiment continued for some days (they gave it some current also, because they were simulating the lightning of those days).
Don’t let this experiment continue, and after Sunday is that result was a dark gunk a molecular gunk was formed. When they analyzed it. It contained a lot of amino acids some nucleic acids Sam complex molecules lipids which make membranes.
So we came to know that in the condition at the beginning of earth 4.5 billion years ago the amino acid was not difficult. Rather in this experiment people think that there was more carbon dioxide etc etc in the early atmosphere.
So this experiment has been repeated several times with different composition, and it is solved formation of amino acids in the early earth condition is not at all difficult meaning that building blocks amino acids etc form easily.
Not only this. We also know now that amino acids are easily formed in hydrothermal vents and these clouds from which stars are formed they also have amino acids. And comets and asteroids which are a part of our solar system (and which often strike our earth and have been striking), they also contain amino acids etc they come to earth from there also.
So the first step (life begin on Earth) of amino acid is not a problem. The idea is this prebiotic before biology soup was very common in the beginning of earth and that is why people are excited. Europa Jupiter’s moon or Saturn’s moon Enceladus have water on it
And hydrothermal vents below the water, and the guess is amino acids etc would be very common there. So the first step of life begin on Earth is not a problem, it’s done.
Second Step of life begin on Earth
But the direct solutions of the next step is not known that is why we are discussing about this, as this is an interesting area of science because it is unsolved. If you imagine billions of atoms are present in simple viruses, so you can think the next part of complex molecule is difficult. But there are many ideas.
For example these lipids have such type of molecules whose one side attracts water and one side repels water. So scientists think these type of molecules as lipids can make a circular form in which the side repelling water is inside, and the one attracting water is on the outside.
This way, basically it can make a boundary or membrane of the cell are link life begin on Earth
And the material inside has an opportunity for different reaction to take place. In the same way, there are many other unsolved mysteries (about life begin on Earth). For example amino acids are in many combination, but life uses only 20 amino acids. The idea that reason for this is that different reaction these 20 can react efficiently. But not only this rather the amino acids that are used are one handed.
What does it mean? Amino acids actually can be left handed or right handed. This mean that in their structure, what is the configuration of the extra molecules on the outside. In nature normally the amino acids besides biology are half left handed and half right handed. But the ones used by life are one time you call them right-handed or left-handed it uses one type of handedness. By the way, this is called chirality this handedness.
This is a puzzle why life uses one type of handedness when in nature both are present. It’s connection is probably with efficiency.
But how to reach there?
As I just mentioned the Miller Urey experiment and the amino acids found in the comets and asteroids in them both handedness are equal. But somehow when life started to evolve it preferred one type of handedness.
Why is this?
Robert Hazen whom I mentioned earlier, because he’s a geochemist, he sees it according to chemistry, according to geology. He thought its connection is with the mineral’s surface. The molecules of this type present on the surface of the minerals are both left-handed and right-handed, and normally they are equally distributed.
Robert Hazen thinks, the handedness of minerals, the left handed amino acid come on the left handed minerals or the other way but the preference is one type of handedness amino acids are on one type of handed minerals. So overall you can imagine these different amino acids are present in billions of billions of numbers on mineral surfaces which are in a separate way. Left-handed amino acids are present on left-handed surfaces and right-handed amino acids on right-handed surfaces.
Now let this experiment at the life begin on Earth continue for several million years and then because changes come randomly, some changes gives you an adventure, for example, one type of this complex molecule give you a little adventure for several then they will survive more. So according to Robert Hazen, tens of millions of experiments were going on at the beginning and because of some variation because of some change in one type of handedness it got an advantage and that is why this handedness became the preference of life.
This is one way of thinking . We do not yet know what exactly happened. This tells you how to think about it and how are scientist approaching it.
Third Step life begin on Earth
And then the third step of life begin on Earth which is the most difficult one and that is the forming of such molecules, which can make copies of itself. Now you must be familiar with the central molecule of life (link with life begin on Earth) that is DNA; Deoxyribonucleic acid;
In this DNA double helix structure is very famous and the bases between them connect with one another. They’re short form is ATCG.
A is always connect to T and C always connect with G. And the sequence in DNA gives information how will the cell work and how will biology go forward.
The problem is , to make copies of DNA instruction are provided by DNA to one protein.
Then another problem arises. If to make copies of DNA you need DNA, then how was the first DNA formed. This comes to chicken and egg problem.
Now this is a challenging question. In this one possible solution is the RNA molecule.
Let me clarify, the question of the origins of life has different approaches . I will give you an idea of one solution, but still we do not know what is the way to original of life according to science. The RNA molecule is a single strand. DNA has two strands and RNA had a single strand and this is Ribonucleic acid.
The idea is RNA can play an important role in the beginning of our life all the beginning of earth. The reason is sometime ago ribozyme was discovered alongside RNA.
This RNA can act like protein meaning RNA has capability to make its own copies. The rybozym was a big discovery, and some people say that if we are talking about the origin of life, then maybe it started with RNA and then DNA took over.
How does this field work?
A good example is a paper published in the journal nature in which the wrote between ribozymes there is a cholesterol RNA’s. These RNA’s can fix amino acids in different chains meaning they can make more complex molecules. This thing is not so important. He noticed that this cluster of RNA’s is actually common in different things. For example archea which is an early life form, mitochondria and in general this type of complex ribozymes is common in the rest of life.
This way he thought, this means (according to him) this type of RNA’s have probably played a central role in the origins of life. We do not know directly whether this is the case or not, but this is one more evidence that supports (the theory) of evolvement of life through RNA.
But before going, I want to give a warning.
If you are successful, sometime is inventing a time machine, then please do not go to period of few billion years of the Earth beginning. That is in is if you reach there, you will suffocate immediately because oxygen was not present there at that time. The oxygen in our atmosphere is actually, as I said earlier due to our life.
That time in the life begin of earth, the blue green algae also called Cyano bacteria, present in huge quantity under oceans release oxygen which reached our atmosphere. And then 2.5 billion years before the life forms on Earth the ones which evolved starting using oxygen.
Now the James Webb space telescope is trying to find out signs of life on exoplanets (other planets) one its aim is to find oxygen in the atmosphere. But even after all of these things, we need to give some respect to single cell life forms, bacteria type, because this single cell life forms have ruled our earth, for most part of its (life begin on Earth) life. And the guess is in the universe, complex life forms maybe scarce but these single cell life forms simple life forms must be very common.