"The world is older and bigger than we are. This is a hard truth for some folks to swallow." --Edward Abbey

I. Defining "life". . .

1. This is difficult or impossible in practice.
   1. There is no single unique character that separates living things from nonliving things -- you always run into a "gray area" (more cases of this will turn up in class later. . .)
   2. It's possible that life in some other part of the universe might exist, but be radically different from the kind that we know -- but how might we know that it's "alive"?
2. However, we can draw up some typical characters that all life as we know it shares.
   1. Chemical composition. All life on Earth is made up primarily of four of the 92 naturally occurring chemical elements: carbon, hydrogen, oxygen, and nitrogen. (These are abbreviated as CHON)
      1. About 95% of a typical organism is made up of CHON.
      2. Many other elements are important in specific roles -- sulfur (S), phosphorus (P), iron (Fe), calcium (Ca), magnesium (Mg), and many more. But these are usually only present in small amounts compared to CHON.
      3. CHON atoms combine to form an enormous number of large, complex molecules, or macromolecules, that have a huge number of functions. More on these later!
      4. All of them interact in liquid water -- a very simple molecule which is indispensable for life in many ways.
   2. Use of matter and energy. All these complex molecules do stuff -- they interact with each other in complex ways.
      1. All living things must take in matter from outside that they use to construct their own bodies.
      2. All living things must also have a source of energy.
      3. In living things, energy is ultimately stored and released by the forming and breaking of chemical bonds which hold atoms together.
      4. Plants and plantlike organisms are able to use sunlight as an energy source, and small molecules such as carbon dioxide as sources for their matter. Organisms that live like this are autotrophs.
      5. Typical animals and animal-like organisms must pick up larger molecules from their surroundings as both a matter source and an energy source. These are heterotrophs.
      6. The use of energy by living things to build up and break down molecules is collectively known as metabolism.
   3. Cells. All organisms on Earth are made up of one or more subunits called cells. (You're made up of roughly ten trillion cells.)
      1. Basically a cell consist of an outer, flexible cell membrane. . .
      2. . . . . a fluid within the cell membrane, the cytoplasm. . .
      3. . . . . and various internal compartments and structures that do various things, called organelles. We will cover organelles in much more detail later, trust me.
         

         Algae magnified about 400x. Click on the image to view a larger image.

         In the alga shown above, each filament is a strand of cells, arranged in single file. Inside each cell are some organelles (visible as green spots) called chloroplasts, where photosynthesis takes place.

      4. Many cells, but not all, contain an organelle that serves as a "command and control center", called a nucleus.
         • This contains the genetic material -- we'll learn later what it is and how it works.
         • For now, know that it's a complex substance called DNA which carries information -- it "tells the cell how to be a cell".
         • Cells that have a nucleus are called eukaryotes; those that don't are prokaryotes.
      5. In a true multicellular organism, cells are specialized for different functions. You have roughly 200 different types of cells in your body.
   4. Reproduction.
      1. Living organisms have the potential to create copies of themselves.
      2. What's more, the copies are near-perfect -- organisms can pass on characteristics of themselves to their offspring. More accurately, they transmit information to their offspring. (You look a great deal like your parents.) This is called heredity.
      3. The copying usually isn't perfect -- changes can and do creep in from one generation to the next.
      4. Since organisms don't exactly reproduce themselves, the door is open for populations of organisms to change over time in various ways. . . but more about that later. . .
   5. Ecology.
      1. No organism lives alone!
      2. The life of every living thing is influenced, both positively and negatively, both directly and indirectly, by a great many other living things.
      3. Ecology is the study of how organisms interact with each other and with their external surroundings, or environment.

1. Biological definition of a species: A set of organisms that can potentially interbreed to give rise to fertile offspring.
   1. This is not a perfect definition, but we still use it frequently; it's convenient and useful in most of the cases that you will encounter.
   2. A little perspective. . .
      1. Humanity forms one species. . .
      2. We're grouped in the Mammalia, which consists of about 4,000 species of mammals (animals with hair, warm blood, producing milk, etc.) Almost half of all mammal species are rodents, and about another fourth are bats.
      3. Add the birds, reptiles, amphibians, and fish. . . you get about 50,000 species of vertebrate (animal with a backbone).
      4. But 50,000 species is nothing. There are over 70,000 species of arachnid (spiders, scorpions, mites, ticks, etc.) There are over 100,000 known species of ants, bees, and wasps. There are nearly 300,000 known species of beetle. . . .
      5. . . . about 250,000 species of land plant, most of which (220,000 species) are flowering plants. . .
      6. . . . in all, about 1.5 million known species of organism are known to exist today.
      7. But that's certainly an underestimate. Previously unknown species are still being discovered.
      8. Current estimates for the actual number of species on Earth usually range from 5 million to 50 million. 10 million is a fairly reasonable, conservative estimate, but no one really knows.
2. It is necessary to have a standardized way of naming species, so that biologists can be sure they're all talking about the same thing.
3. We also have to have a stable, standardized way of classifying living things -- with 1.5 million named species to keep track of.
   1. What's this fish called?
      

   2. Some of you would call it a "bluegill", some a "bream", and some a "perch". In other parts of the country, "sunfish", "sunperch", or "panfish" is the usual name.
   3. And that's just in English! In Russian, for instance, this is a sinezhabernyy solnechnik. This could get confusing fast, especially since science very often involves international collaboration.
4. An 18th-century Swedish botanist named Carl Linnaeus developed a convention for naming and classifying species that we still use (with some modification). (Here's some more information on Linnaeus, for anyone who's interested. . .)
   1. Greek philosophers such as Aristotle had said that, to classify something, you had to do two things:
      1. Identify the general kind of thing it is (its genus, plural genera) -- what makes it similar to other things?
      2. Identify what makes it different from all other things like it (its species, plural also species)
   2. Linnaeus used that philosophy to devise a system of binomial nomenclature (binomial = "two names"):
      1. A species' complete name is its genus name followed by its species name.
      2. This name is in Latin, and should always be written in italics or be underlined.
      3. Whoever first discovers a new species, and officially describes it, gets to name it. This is still going on today.
      4. Look at the bluegill again (or is it a perch?)
         This fish has exactly one unique Latin name -- Lepomis macrochirus.
      5. Any biologist, no matter where she's from or what her native language is, will recognize and understand that name, or at least will know where to look it up.
      6. The bluegill sunfish (Lepomis macrochirus) is quite similar to other fish, which we classify in the same genus. . .
         
         Lepomis microlophus (the red-ear sunfish)

         
         Lepomis cyanellus (the green sunfish)

5. Linnaeus's other innovation was the use of hierarchical classification.
   1. Species are grouped together in genera. . .
   2. then those may be grouped in more inclusive groups, and so on. (Each of these groups is called a taxon.)
   3. Usually, each taxon is assigned a rank depending on how inclusive it is.
   4. Starting from the top, the basic taxon ranks are:

      Kingdom

      Phylum

      Class

      Order

      Family

      Genus

      Species

   5. Sunfishes (genus Lepomis) resemble bass (Micropterus salmoides is the largemouth bass, Micropterus dolomieui is the smallmouth bass) in many ways
      
      Micropterus salmoides, the largemouth bass

   6. Thus the genera Lepomis, Micropterus, and some others all go in the same family, whose Latin name is Centrarchidae.
   7. The family Centrarchidae goes together with many other families of fish into an order called Teleostei, consisting of fish with symmetrical tails and mouths . . .
   8. The order Teleostei goes together with some others into a class called Osteichthyes, consisting of all fish with rayed fins and a bony skeleton. . .
   9. The class Osteichthyes goes with some others into the phylum Chordata, which includes all animals with a nerve cord running down the back. . .
   10. And the phylum Chordata goes with some others into the kingdom Animalia, which includes all multicelled organisms with complex embryonic development, collagen proteins, etc.
   11. Here are the taxa into which our species, Homo sapiens, falls:

       Kingdom: Animalia (heterotrophic organisms with many cells of many specialized types, embryonic development, collagen proteins)

       Phylum: Chordata (animals with a spinal cord)

       Class: Mammalia (chordates with hair and milk glands)

       Order: Primates (mammals with five grasping fingers and toes, good vision, relatively large brains, forward-facing eyes with complete bony eye sockets)

       Family: Hominidae (primates lacking tails)

       Genus: Homo (hominids with upright posture, tool use)

       Species: sapiens (Homo with brain size about 1300 cm³)