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As a scientist, how would you explain how the Earth was created after the Big Bang occurred? Where did animals, plants the human race come from?
Answer 1:

The creation of the universe took place about 15 billion years ago!!! See the Scienceline web site for more on that.

The EARTH formed the same time the sun formed, about 4.5 billion years ago (4.5 Gyear) the big G stands for giga which means multiple what follows by 1,000,000,000.=1 billion.

At any rate, about 4.6 billion years ago, there was a giant MOLECULAR cloud in space. This is an area of space that contains Hydrogen and Helium in concentrations of about 10 to 100 atoms per cubic centimeter .... This giant molecular cloud was rotating slowly and was at a Temp of about 20 to 50 K (very cold indeed).

This giant molecular cloud can have one of two histories: the cloud can dissipate or disperse, OR the cloud, because it has sufficient mass, will gravitationally collapse. If the cloud collapses, then things start to happen quickly... the end result is a region within the cloud where concentrations of mass developed in a restricted region. This region is called a STELLAR NURSERY...it is a region in which anywhere from tens to THOUSANDS of stars begin to form. These objects are called embryo stars.... if we focus now on ONE of these, we see that gravity continues to collect matter into a smaller and smaller region of space. The original rotation that was present will cause the PROTOSTAR to SPIN UP (take on a higher rotation rate). At the same time the nearly spherical protostar will begin to throw stuff out in its equator and a circumstellar DISK will form. This is a pancake sized disk surrounding the now spinning protostar.

PLANETS form from this disk (the circumstellar disk) as dust particles in the disk, they hit one another and stick to each other gradually increasing in size from rocks to boulders, to house size rocks up to 100 km size aggregates, which we call PLANETESIMALS. These planetesimals collide with one another sometimes fragmenting into many more pieces, but in the long run the collisions are not energetic enough to break them apart. Instead the pieces "stick"; we call this process COLLISIONAL ACCRETION.

The amount of time to go from the flattened disk of gas and dust surrounding the protosun to a collection of planets, asteroids, and comets is about 50 to 100 million years (50-100 Ma).

In the end stages of collision accretion, some rather large collisions take place. When the earth was 70% its present mass, it was hit with a MARS-SIZED planetesimal and the result of THAT collision produced the earth's MOON.

If you are interested in this subject look up info on meteorites and on the ORIGIN of the solar system.... use these key words on the web and you will be inundated with information: METEORITE, planet evolution, origin of solar system, solar nebula.


Answer 2:

Wow, you sure asked a toughie! This question, which seems pretty simple, has been a source of very lively debate in the general science and biology communities for a long time now. Even NASA (the National Aeronautics and Space Administration) is interested in the answer to your question. Since the answers aren't really known, scientists can only speculate about how life came to be after the big bang. There are many theories (including a few religious ones), and I'm sure I haven't heard them all. I will give you my idea of what could have happened, but I encourage you to read more about it. I would try searching the web using the phrase "origin of life" or reading a recent (post 1980) college textbook on introductory biology. (I like "Life, The Science of Biology", by Purves, Orians and Heller. If you want I can send you photocopies of relevant chapters.) If you are still interested in how life appeared and evolved after doing some general reading, I would encourage you to read books like "The Lives of a Cell", by Lewis Thomas. It's a great book, and a classic in the field. Also, Darwin's book on evolution, called "The Origin of Species" might be a good place to start. If parts of these books are too difficult, try reading a few chapters with your teacher as a class assignment.

Most scientists believe that just after the earth was formed 4.5 billion years ago it was a very nasty place, and not at all capable of supporting life as we know it. First of all, the atmosphere was different than it is today. Initially the earth's atmosphere had no oxygen, so the first organisms to appear had to have been anaerobic (able to live without oxygen). The earth's early atmosphere also had no ozone to block out UV rays produced by the sun, so not only was there a lot more UV hitting the earth, but the UV rays included a type of UV that is more harmful than the types we experience on earth today: UVC. Scientists believe that the early atmosphere was also full of harmful radiation. The climate on earth was much different than it is today. It was very hot due to volcanic activity, so hot that most plants and animals alive today would die. This is why many scientists believe the first organisms to appear on earth were not only anaerobic, but also able to withstand high temperatures and high UVC and radiation exposure. This would require unbelievably tough cell membranes. There are certain types of bacteria alive today that can live no matter how much UVC and radiation you expose them to, and there are bacteria that can grow at temperatures of 113 degrees
Celsius (water boils at 100 degrees Celsius). For this reason, and because they are single cells, it is thought that bacteria were the first organisms to evolve on earth.

Just how a living cell was generated from non-living material, and was able to replicate itself exactly so that it could reproduce, is a mystery. Many scientists have tried to recreate this "spontaneous generation" of life under controlled conditions similar to the earth's early atmosphere, but have not succeeded (one example was the
Miller/Urey experiment). The idea is that a sudden input of energy (e.g., lightening) caused certain molecules to rearrange randomly into a living cell. My thought is that living cells probably appeared and died several times independently before cells were able to reproduce themselves and spread. Because viruses are the simplest organisms capable of reproducing themselves, viruses may have been the first forms of life (if you consider them to be alive). However, since viruses depend on the cells of bacteria (or other organisms) to multiply, scientists who study them believe they evolved after
bacteria.

So, how did oxygen appear in the earth's atmosphere? Even today oxygen in the atmosphere has to come from photosynthesis, which only plants, algae and some bacteria can do. The first organisms to produce oxygen on earth were a type of bacteria called cyanobacteria. Scientists have found cyanobacteria fossils (called stromatolytes) from 3.8 billion years ago, the earliest evidence of life that we
have. These early cyanobacteria lived in the earth's ancient oceans, which were probably hot and very salty, maybe even acidic. Even today, cyanobacteria are able to live in the most extreme environments: frozen ponds and snow in Antarctica (Antarctica has very high UV levels), hot hydrothermal pools in Yellowstone and salt
ponds.

Once oxygen appeared in the atmosphere, animals could now survive, multiply and evolve. Bacteria are the only organisms I know of that are capable of living entirely without oxygen, so photosynthesis needed to occur before animals could survive. We see the first evidence of non-bacterial life in fossils from about 2.5 billion
years ago. The first animals to appear on earth lived in the ocean. They were single


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