Click here for a
table of processes
leading to human
life on Earth.
(At left: deoxyribonucleic acid, or DNA. This molecule, upon which all life on Earth is based, is a chemical arrangement of atoms found in abundance in the universe. It is not unreasonable to suppose that this molecule, and others like it, assembled itself naturally in the conditions existing on the early Earth.)
Lesson: cosmic evolution
So far in this course, we have worked our way from small to big, from local to distant. We moved from eclipses, seasons, and global warming out to the rest of the solar system, the stars, the galaxies, and the entire universe. Now we take one final step grander than all of that: we add time to our picture of the universe.
Cosmic evolution is the study of the universe as it has evolved over the history of the universe, which makes it a useful framework for studying the origin of life.
We identify 7 major phases in cosmic evolution, listed in chronological order:
- Particulate. In the early big bang, the entire universe was all about the interactions of particles. Particle collisions and transformations are responsible for the synthesis of the light elements (BBN) and the separation of the CMB radiation from neutral matter. Before structures like galaxies formed, there were only particles.
- Galactic. Dark matter particles accumulated into gravitating clumps which pulled in normal matter, forming large, isolated systems that became galaxies.
- Stellar. Within these early clumps of dark and normal matter, smaller clumps of normal matter fragmented out, forming stars, star clusters, and star systems (like our solar system). These stars evolved over time and often exploded as supernovae, scattering new elements (heavier than helium) throughout the galaxies. New star systems formed from this enriched material.
- Planetary. Planets formed around new stars made from these heavier materials like iron, silicon, carbon, and water. Our solar system, and especially Earth, shows us how the young planet experienced volcanic and plate tectonic activity, atmospheric changes, etc., which still go on today (although slower). Mars, for example, appears to have completed such activity. Impacts from comets and asteroids delivered certain materials, including water and amino acids, to the planet's surface.
- Chemical. On the surface of the Earth, energy input from the Sun, weather, and geothermal sources stirred the oceans, mixing chemicals into new forms. The chemical mixing attempted many combinations of atoms, and organic molecules probably appeared quickly - amino acids, fats, sugars, and ultimately perhaps DNA.
- Biological. Different arrangements of organic molecules become organized into distinct self-contained cells. Exchange of DNA between cells (sexual reproduction), genetic mutation, and natural selection gives rise to new life forms, new behaviors, and intelligence.
- Cultural. Among our intelligent species, the fastest type of evolution today is cultural rather than biological. Technology, politics, and social changes dominate our progress. Will we become an advanced Galactic civilization? Will we destroy ourselves? Will we create new life forms based on artificial intelligence, which might surpass us?
Together, these components form our current understanding of cosmic evolution. Look over the above list critically, and try to recognize its general character. The systems of importance, by this description, progress from enormous regions of space to individual planets, then to an individual component of our biosphere (us). The systems thus go from large to small, but at the same time from simple to complex. Tracking our current cultural evolution state, in which we study our origins (for example in this class!), back to the early particulate state, we see that we are (as Carl Sagan once put it) basically just hydrogen which has learned to look at itself!
Does any of this help us understand who we are and why we're here? Would some other universe evolve in the same way, from the particulate to the cultural development of intelligent civilizations? Does this progression seem standard enough that we should expect similar civilizations on other worlds?
Lesson: the origin of life
The following is a scientific theory on the origin of life on Earth. As with any theory, it is not proven, nor can it be. Parts of this theory can be tested by experiment, and they do hold up to such scrutiny - at least so far. Two points deserve to be made at this point:
- This theory is incomplete: there are gaps and misunderstandings in the picture we are about to describe. The big bang, for example, is a far better developed and tested theory. We are going to examine science's best ideas on the origin of life; they are consistent and plausible, but this is a work in progress.
- None of the following even begins to address why we are here - only how we got here.
Now we will investigate some general features of the origin of life on Earth.
First, consider the Earth itself. It probably formed from planetesimals, containing heavy elements formed in large stars. In the molten Earth, heavier objects tend to sink. So we have an iron core, and a lighter crust made of silicon and magnesium and carbon and other elements. On top of that is water (oceans), and lighter still, the atmosphere above.
That is, if you just imagine where all the elements would go in forming a planet like this by gravity, they ought to end up pretty much where they are.
We're moving from planetary to chemical evolution.
Two sources of organic (carbon-based) molecules existed on the young Earth (we don't know which is more important):
- Comets and asteroids have been measured to contain significant quantities of organic material, which would have fallen from space onto the Earth by means of impacts, especially when the solar system was young.
- Organic molecules would have formed spontaneously on Earth. This idea was tested in 1953 by two scientists for whom the experiment is named. The "Miller-Urey experiment" used an apparatus as shown below to simulate the energy and matter state of the early Earth, and found that amino acids, fatty acids, and sugars formed spontaneously! Nitrogenous bases (A, C, G, T, U) that make up genes all formed spontaneously as well, in more sophisticated experiments that followed.
In the Miller-Urey experiment, electrodes simulated lightning and a heater simulated geothermal energy. Water containing appropriate minerals simulated the early oceans, and a collection of available gases simulated the young Earth's atmosphere. In other words, these scientists constructed the primordial soup of our newly formed planet, and the energy sources present (e.g., lightning and geothermal). They found that organic matter develops naturally in this context:
- adenine (nitrogenous base)
- cytosine (nitrogenous base)
- guanine (nitrogenous base)
- thymine (nitrogenous base)
- uracil (nitrogenous base)
- amino acids
- sugars
- fatty acids
Since then, other origin-of-life experiments have further enhanced our knowledge in this area.
Solar ultraviolet rays (before the formation of the ozone layer) was another important energy source for manipulating chemical bonds.
Polymerization was an important next step. That means that basic ingredients needed to be strung together end-to-end to form long chain molecules:
- nitrogenous bases, phosphates, ribose sugar ---> DNA and RNA
- amino acids ---> proteins and enzymes
- fatty acids ---> lipids
- sugars ---> polysaccharides and starches
- ribose sugar, adenine, phosphates, UV radiation ---> ATP
DNA carries genetic information. RNA and proteins carry out cell functions. ATP (adenosine triphosphate) is used by many cells to extract useful energy from nutrients.
Exactly how polymerization happened is unknown for the production of DNA and RNA. This has not been reproduced in the lab. It is an important part of origin-of-life science which has not been solved yet. The problem breaks down like this:
- In living cells, DNA tells the cell how to form RNA, and that RNA tells the cell how to make the proteins it needs to maintain the structure and function of the cell. That is, DNA ---> RNA ---> proteins.
- But in order to make DNA, or for DNA to self-replicate, you need enzymes, which are a type of protein. That is, protein ---> DNA.
So we have a problem: you need enzymes to make DNA, and you need DNA to make enzymes. But some good progress is being made in researching this problem, and a more comprehensive story is slowly emerging to describe the origin of these important biological molecules. No solution is complete yet, but one could be someday soon. In the meantime, let's ignore the problem by saying that somehow DNA, RNA, and proteins all formed. What's next?
Proteins are able to collect into membranes for droplets of organic material. The process proceeds automatically in nature (verified in experiments), forming droplets called "coacervates" as pictured below. These droplets contain higher concentrations of organic matter than the surrounding fluid. That is, the world's primordial soup organizes itself into protocells containing more DNA, sugars, etc., than the surrounding soup.
These droplets are not yet cells, but they're getting close. Scientists have not yet constructed a living cell, but imagine for a moment that it is possible. We know about half of all the processes which take place within an E. Coli bacterium - but what if we learn all of them? In principle, then, we could build a living cell.
Is that unrealistic? If we assembled all the correct molecules into a bacterium, would it be alive? If a billion years of random nature at work can do it, then is it crazy to think that intelligent beings can do it too? Is there some "spark of life" that we are unable to recreate?
Protocellular droplets would occasionally swallow other droplets - similar to organelles inside our cells today. In fact, it is widely believed that mitochondria, which allow our cells to use energy, and chloroplasts, which allow plant cells to use photosynthesis, originated in this way. A larger protocell "swallowed" a smaller one capable of photosynthesis, for example.
When large droplets break apart (from rough seas, say) we know that the proteins will reassemble into more coacervates, making "daughter" protocells with the same DNA as their "parent" protocell. This is heritable reproduction.
Droplets can take in particular nutrients through their membranes from the outside, and then "spit out" parts they don't need. This constitutes growth (or metabolism).
Stronger droplets survived better, and their daughters would be similarly strong because they contain the same genetic material. That is, natural selection is working on a chemical level (rather than a biological one).
So we have objects that grow and reproduce: this is a very basic definition of life!
Lesson: biological evolution
Suppose now that living cells were created as described above. A number of important changes followed by natural selection. For example:
- Some cells could make their own ATP. Other types of cells died when the oceans ran out of freely floating ATP. (ATP-based energy generation is now the basis of the mitochondria organelles in modern animal cells, like ours.)
- A cell that acquired its energy from sunlight became part of a more complicated cell - the smaller piece evolved into "chloroplasts" which carry out photosynthesis in plants now. This process first occurred in early algae, creating oxygen molecules in our atmosphere.
- An oxygen atmosphere was toxic to many early cells; most of those cells died. But an oxygen atmosphere (O2) aided energy generation in those few cells which could use oxygen (like ours).
- An oxygen atmosphere allowed the formation of ozone (O3) which protected new organisms from being killed by solar ultraviolet rays. This made life on land possible.
- The early biosphere had algae (producers of oxygen and sugars, etc.) and bacteria (consumers of these ingredients).
- Some cells developed the ability to move around to find food.
- Life evolved into more complex forms. This process was aided by climatic changes and major Earth-impact collisions, which caused extinctions of certain life forms, allowing others to survive with less competition. For example, if a comet or asteroid strike hadn't wiped out the dinosaurs, mammals probably would never have become so successful, and humans probably would not have evolved. In a fair fight, reptiles ruled over mammals.
- Societal behavior may be a naturally selected trait, important to hunting. Groups of hominids (early people) which stuck together would survive better than those who fought among themselves. A sense of morality, and service to the community, may even have arisen this way (protect your neighbor and love your family, etc.). Perhaps this sort of "behavioral natural selection," from long ago, became the basis for our legal systems.
- Intelligence, emotion, and consciousness result from a certain quantity of brain cells - increasing the number of neurons increases the capacity. In this sense humans don't have better brains than other animals, but rather we have more brains. However, human intelligence remains a minor weak point in the theory of evolution today. We don't yet know exactly why evolution would create such big brains, although several interesting possibilities have been put forth, mostly involving advantages in socialization of primates and homonids.
All of this is based on Darwin's natural selection mechanism, which says that
- if you have a population with variations, and
- if you have heritable traits (daughters look like parents), then
- survival of the fittest directs which traits will emerge
Suppose that our behavior is the result of natural selection processes. Emotions thus help us in some way. For example, in the past, anger might have helped us to become more aggressive in battle situations, to defend our territory and our families. This would clearly aid the survival of early humans.
But what role does anger play in modern society? Is it ever helpful? Do our evolved-in aggressive tendencies conflict with the civilized world?
Consider the fact that our technological and cultural evolution is faster than any biological evolution which might soften our aggressive genes. Does this place us on an inescapable route to self destruction?
Lesson: searching for Martians
Earlier in the history of the solar system, Mars was more temperate. We see evidence for rivers of water that used to exist there. We see water ice frozen on the surface, and expect more underground. If Mars was warmer in the past, when it's internal heating (like geothermal on Earth) was more active, then it may have been a lot like Earth is now.
And if bacterial life forms easily (that is, if fl is near 1), then it is reasonable to speculate that it once existed on Mars (and maybe some still exists!).
In 1996, President Bill Clinton is reported as having whispered to one of his aides, "only seven people on Earth know this - NASA has found life on Mars!"
In the movie Contact (about receiving a message from aliens), real footage of Clinton is shown as he gives a press conference about extraterrestrial life, actually regarding this discovery from Mars.
We have sent several spacecraft to land on Mars, and none has found evidence of life - although that wouldn't be easy to find for a robotic spacecraft.
The evidence was actually found on Earth, in Antarctica. There, we found a meteorite with pockets of Martian air trapped inside. The composition of this air tells us that the meteorite is from Mars. Some asteroid or comet impact on Mars long ago blasted some Mars rocks into space, and occasionally they land on Earth; this is one of them.
The meteorite is called ALH84001 (because it is the first meteorite [001] to be found in 1984 at the Allan Hills site).
The meteorite contains the following lines of evidence for former life on Mars:
- Carbonate globules similar to ones formed by bacteria on Earth
- Magnetite and pyrolite minerals, also as made by Earth bacteria
- Polycyclic aromatic hydrocarbons, often created organically
- Oval and tube structures that look like fossilized life forms
Here is a picture of one of these alleged fossils from ALH84001:
And for comparison, here is a collection of the oldest fossils on Earth, from what is now Australia, radioactively dated as 3.5 billion years old:
In the above picture (from Earth), the segments are chains of individual cells strung together, as modern algae and bacteria do on lake beds.
The four lines of evidence above would not be compelling on their own. That is, any one of the four, by itself, would not be considered to be real evidence of life on Mars.
There are two opposing camps today:
- As Carl Sagan once said, "extraordinary claims require extraordinary evidence". By this rationale, the ALH84001 evidence is not convincing enough to say anything so grand as the discovery of extraterrestrial life.
- On the other hand, "Occam's razor" says that "all things being equal, the simplest explanation tends to be the right one". By this rationale, finding all four types of evidence in the same rock can be most simply explained by saying that a single type of bacterium created them all.
Realistically, we can't be sure which view is correct. Only a manned Mars mission is likely to help here - you really need people to hunt for, and handle, potential fossils. In time, this question will be answered conclusively.
Such manned Mars missions have been designed, but not yet funded. The cheapest is called "Mars Direct", with a similar expense to the 1960's Apollo Moon program. The Mars Direct plan would take about 2.5 years round trip, involving about 1.5 years for a crew of 4 to live on Mars. Over time, the Mars Direct method would build up a permanent staffed Mars base. You can read about it, from its inventor's (Robert Zubrin) article in Scientific American, March 2000, here. (If you have trouble with this link, go to www.sciam.com, and then "past issues" to get the article.) NASA has made its own mission plan, which costs a little over twice as much, and involves a crew of 6 (not 4), but is otherwise quite similar.
Lesson: SETI
The Pioneer 10 spacecraft, launched in the 1970s, is leaving the solar system for interstellar space. On it, we mounted the following plaque:
The plaque is intended for intelligent aliens to find. It shows a man and woman, next to the Pioneer 10 spacecraft (for size comparison). It shows our solar system, a hydrogen atom making radio emissions, and a starburst pattern giving the locations and pulse frequencies of many pulsars nearby Earth (this guides the aliens to where we live, because they can presumably also identify where these pulsars are).
Apart from this plaque, the usual approach to SETI (the Search for ExtraTerrestrial Intelligence) is to listen only, and not broadcast. We want to know a little about the aliens before we tell them how to find us.
SETI is a very difficult game: you can search for radio communications from other civilizations in many different frequencies and many different directions in space. There are around 10,000 stars visible from Earth with the naked eye alone, and about 100 billion stars in our Galaxy in total - so with radio telescopes, there are prohibitively many targets and frequencies to search.
One approach has been to search in the "water hole" radio frequency. This is right between the natural emission frequencies of the H (hydrogen) atom and the OH (hydroxyl) molecule. For one thing, this frequency is particularly radio-quiet in our Galaxy. For another, perhaps other life forms will also share our appreciation for the importance of water, and choose to broadcast there, if they want to be found.
You can personally help with the SETI search if your computer is on the internet, by downloading a special screen saver which will analyze SETI radio data while you're not using your computer. (See http://setiathome.ssl.berkeley.edu/ if you're interested.)
Finally, it's worth noting that we are sending radio signals out into space right now, and have been for about 70 years. Therefore any alien civilization within ~70 light years could have heard us by now if they know what to listen for, like our SETI program does.