A Brief History of Life

the first 15 billion years

Prologue: Nature of Science - Before Once Upon A Time

Science is both a body of knowledge and a process that gains information through observation, experimentation, and inference. Science follows rules, and its results are tested, revised, and retested until a great deal of confidence is reached in a particular conclusion (and even then it is still open to peer review and skepticism as new evidence arises - it is a process that corrects itself). These well-substantiated conclusions may then be referred to as a theory, as in the theory of evolution. Science answers questions such as "what is there?", "where did it come from?", and "how does it work?", all in an attempt to learn more about our lives and our universe. The key is that science relies on evidence derived from observation, experimentation, and logical inferences. When faced with multiple possible answers, science prefers the most parsimonious explanation. Science is not a democratic process - truths are not found by popular vote, but based on evidence - the number of people who 'believe' in something is irrelevant (flat earth, geocentrism).

Chapter 1: Kaboom - Let's Get This Party Started

In the beginning there was a dot that was the size of, well... a dot. Sort of like the dot at the end of this sentence. All the energy of the universe was stored in that dot and one day it exploded with what one could aptly call a big bang. The universe expanded rapidly as energy converted into matter and that matter started to coalesce as the force of gravity took hold. A ‘short’ 10 billion years later the Earth was formed in a place in our solar system that was conducive to what would happen next. (rare earth factors) As the earth continued its development during the next billion years or so, the right environment arose to start the wondrous process we call life. The order of these upcoming events are determined by relative and absolute dating methods, such as radioactive dating. Life may have begun at the bottom of the ocean at hydrothermal vents, or it may have begun in the ‘primordial ooze’ at the surface, or it may have started elsewhere in the universe and been deposited here via a meteor or comet. However it happened, life started small – likely as some self-replicating molecule, possibly RNA, along with other types of organic compounds.

Chapter 2: There's No Place Like Home

These molecules needed a protective ‘house’ and thusly the first cell was born. The cell membrane eventually was made of a phospholipid bilayer that was selectively permeable. It only allowed certain compounds in and out, typically via diffusion. As cells grew in complexity, other items were moved in or out via active processes requiring energy, typically in the form of ATP. Early cells were prokaryotic with eukaryotic cells evolving from them. The membrane bound organelles in eukaryotic cells each perform a function for that cell. Two of these organelles, mitochondria and chloroplasts, might have been prokaryotes themselves and formed a symbiotic relationship with other prokaryotes. Furthermore, eukaryotic cells use DNA as their genetic material to instruct the cell in what to do. For protection, the DNA is housed in the nucleus of the cell and sends messages out to the cell in the form of messenger RNA (mRNA) through the process of transcription. This mRNA codes for proteins at the ribosomes via translation and these proteins perform the functions of the cell. Proteins help to make cell structures, transport channels, enzymes, and messengers that communicate with other cells in multicellular organisms. These multicellular organisms were the next step in evolution. This allowed cells to differentiate, which means to specialize in a particular task, and create a larger organism with a greater range of function and ability. This gives life 'levels of organization': cell - tissue - organ - organ system - organism. Of course, to do this, cells must be able to reproduce.

Chapter 3: Two's Company and Three's a Crowd, but a Million is Just Getting Warmed Up

Cell reproduction, or mitosis, is an asexual process that produces two offspring that are identical to their parent cell. In cell division, the cell undergoes various changes which are divided into 6 stages collectively known as the cell cycle. The stages are: interphase, prophase, metaphase, anaphase, telophase, and cytokinesis. Put simply, during mitosis the cell doubles its DNA and organelles and then divides in two. We use imaging technology, like microscopes, to view cells and cell division. Like other cell processes, mitosis requires energy.

Chapter 4: You Are My Sunshine - Now Get in My Belly

Organisms can obtain their energy either autotrophically or heterotrophically. Autotrophs make their own nutrients by using the energy of the sun (photosynthesis) or the energy in other chemicals (chemosynthesis). Photosynthesis takes water and carbon dioxide and, using the sun’s power, converts it into glucose and oxygen. This process occurs in the plant’s chloroplasts, which are in their leaves. The main pigment used by plants to capture the sun’s energy is called chlorophyll. In the chloroplasts various chemical reactions occur in a cycle called the Calvin Cycle. The plants use the Calvin Cycle to convert energy from sunlight into glucose. The plant then uses the glucose as energy or to build plant structures. Heterotrophs, on the other hand, obtain their energy by eating autotrophs or other heterotrophs and breaking down the compounds ingested via the digestion process. The most important energy compound being glucose, which heterotrophs break down to form ATP, which the cell uses to power reactions. Depending on the type of heterotroph you are, you may break down glucose using anaerobic glycolysis or you may add the Kreb’s Cycle and electron transport chain to obtain even more ATP. The ATP produced is used by all cells to power other chemical reactions that allow a cell to perform its function. Excess glucose is stored in a form that is particular to the organism. Plants store excess glucose as starch while animals store excess glucose as glycogen and fat.

Chapter 5: The Birds and the Bees

Now that we have organisms capable of surviving and reproducing asexually it seems a good time to add some creativity to the mix. Thus the process of sexual reproduction came about. In sexual reproduction both parents form gametes via meiosis and these gametes come together to form an offspring that is not identical to either parent. This sexual reproduction allows for greater variety through genetic recombinations and therefore greater evolutionary possibilities.

Chapter 6: Darwin, a Tortoise, a Beagle, and a Finch Meet on an Island...

Evolution is a process that works through natural selection allowing for ‘survival of the fittest’. Random mutations occur to an organism’s genetic material which are either beneficial, harmful, or have no effect on that organism’s survival in their particular environment or niche. If that mutation causes a beneficial adaptation, and it is heritable, then it is passed on to future generations and increases the likelihood of survival of all organisms containing that mutation. If it is harmful, then it is selected against and dies out of the population. Neutral mutations may or may not spread through a population due to genetic drift. As time goes by, mutations accumulate and organisms may become significantly different compared to their ancestors. Eventually, speciation events will occur where enough changes have accumulated that the organisms can no longer produce fertile offspring with their ancestral species. The mutations may be in a specific nucleotide , such as point mutations or frameshift mutations, which will lead to different versions of a gene that are each called an allele. Alternatively, a mutation may be the insertion, deletion, translocation, inversion, or duplication of a whole gene or set of genes. Furthermore, mutations may include having an extra copy of, or missing, a whole chromosome. All of these mutations affect the resultant proteins produced and thus the function and abilities of the cells of that organism. This, in turn, may lead to new organisms and new species. Biologist use cladistics to create phylogenetic trees to show evolutionary ancestry. These trees were originally created by observing similar homology amongst organisms via their phenotypes. With the completion of the Human Genome Project, biologists now include genetic data to create these trees based on genotypes and the sequencing of various organisms’ DNA. New branches of study have arisen to analyze and make sense of the vast amounts of data available today including biomathematics, computational biology, and bioinformatics. This information also aids genetic engineers in altering existing species to meet a desired purpose. The changing of an organism's DNA by humans raises various ethical issues.

Chapter 7: Why Don't I Look More Like You?

Genetically, humans have 46 chromosomes that work together in pairs (23 pair). 22 pair are autosomal and 1 pair are the sex chromosomes (XX or XY). Chromosomes are of different lengths and contain different numbers of genes on them. There are a total of approximately 25,000 genes spread amongst the 23 pair. Each pair of chromosomes works together to control for the same traits, such as hair color or blood type However, each chromosome in the pair may have a different version of a gene (allele), such as brown or blond, for that particular trait. The two alleles work together in one of the following ways: dominant/recessive, codominance, or incomplete dominance. Furthermore, there may be only two alleles for a particular trait in a population, or there may be multiple alleles that relate to each other in the various ways mentioned above. The 'ABO' blood types are the classic example of multiple alleles. Also, a trait may be polygenic, which means that there are several genes on the same or different chromosomes that work together to control a single trait, each pair with their own versions of alleles and own type of dominance relationship. The sex chromosomes carry more genetic information than just sex determination, and the X and Y chromosomes do not carry the same genes. Because of this, some traits are linked to, or more likely to occur in, a particular gender. Other creatures have a different number of chromosomes with different genes and alleles. All of this creates the incredible variation that we have in this world.

Chapter 8: Can't We All Just Get Along?

Eventually, mutations spread through and affect a whole population. This, in turn, affects other populations of organisms due to various interactions among different species and their relationships through the food chain. Populations of organisms interact with one another in various ways, setting up an ecosystem. These interactions can include symbiotic relationships such as mutualism, commensalism, or parasitism. Alternatively, there could be a predator/prey relationship between two species such as fox and rabbits. These interactions and available resources will cause each population to have its own carrying capacity. Additionally, matter in the ecosystem is recycled via the carbon, water, mineral, and nitrogen cycles. Energy is also is recycled, but much of it is lost as heat; thus, the number of trophic levels are limited. All of this leads ecosystems to establish an equilibrium that can be impacted by natural processes or human endeavors.

Chapter 9: Why Don't You Pick on Someone Your Own Size?

One major item that affects a population is disease. Epidemiology is the study of the cause, distribution, and control of disease. These diseases are usually associated with a particular bacteria or virus. Normally, our immune system protects us from these microbes without us even realizing it. However, when we do not eat well, exercise, get enough rest, or are over-stressed, our immune system weakens and these microbes are given a chance to take hold and multiply in our bodies - mitosis in a big way. When this occurs our bodies respond by increasing the temperature (fever) and flushing the system (runny nose, cough, sneeze, diarrhea, watery eyes, etc....) as well as producing antibodies to attack the invader. If we are too run down we may need external assistance in the form of antibiotics for bacterial infections or antivirals for viral infections. If a disease is spreading through a population an outbreak is said to be occurring. Disease spread may be considered endemic or epidemic. If it is a viral outbreak, vaccinations may be given to help control its spread. Epidemiologists try to model various vaccination strategies to obtain the best results.

Chapter 10: Will We Ever Learn?

The spreading of disease is becoming a larger world problem due to the ease of travel and the greater interaction of people all over the globe. Diseases that are spreading around the world are said to be pandemic. Other problems we face as a global society include the lack of clean drinking water, global warming, biomagnification, and species extinctions (among others). These problems can be attributed to, or exacerbated by, the overarching problem of over-population as we put too much stress on our planet's ecosystems.

Note: These chapters do not correspond to chapters in your book - or any book, for that matter. This information is provided as an overview to give you the Big Picture of biology. As we go along, we will be adding more details and associated resources to this text. This should be considered a first draft and, just like science, it may change as new information is acquired and current information is reviewed.

Page last modified on Wednesday August 18, 2010 12:48:09 EDT