Life

leftrightleftrightleft said:

Firstly, what compels life to keep living? Why does a blood cell in my body (a component of life) continue its rounds carrying oxygen? What compels the bacteria currently infecting my throat to continue infecting? Quite simply, why don't living things just stop living?

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Because if it did it wouldn't be here. Think about it. You have two groups of organisms, one with a strong desire to live, the other without. Which is going to be here in a billion years?

it's not that "life" has some intrinsic (or extrinsic) desire to exist, it's that the trait of "desiring to exist" out competes the alternative. "Wanting" to survive is a strong fitness trait. Having said that, blood carrying oxygen is a fairly simple biochemical reaction of heme to differing oxygen gradients.

We're here because we're here.

Secondly, are viruses considered life?

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Yes, no, maybe.

Thirdly, is there any theory out there as to how sexual reproduction evolved? Did some proto-sperm swimming in the primordial sea one day bump into a proto-egg and by some chemical miracle their DNA combined?

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Nothing like it. Simple organisms, such as yeast, engage in sexual reproduction by exchanging genetic material, which is all sexual reproduction is. Since sexual reproduction confers a number of advantages, including the ability to propagate genetic subsets through a population as well as cleanse accumulated mutations, it's a trait that was kept and as life became more complex sexual dimorphism arose (the concept of male, female, sperm, eggs)

Bacteria exchange genetic material all the time, it's just not officially part of their reproductive cycle.

Fourth (and the longest; related to the third): Evolution is also described for simplicity sake as a basically continuous process. You'll hear scientists say that dinosaurs "evolved" into birds, as if this is a continuous, flowing process like pouring water into a cup. But isn't the real world actually terribly discontinuous? As an example, take a multicellular organism. At some discrete point in history, there had to be only unicellular organisms. And then at some discrete point in history, there had to be multicellular organisms. Where is the continuous go-between that would allow one to get to the other without making a discrete "miraculous" jump?

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This is known as the sorites paradox, which you can find on wiki, but paraphrased it means that just because you cannot define a discrete boundary between two states does not mean those states do not exist. Everyone will agree there is a difference between having a beard and not having a beard, noone will agree on exactly how many hairs are needed for it to be called a beard.

There was a recent experiment done in which a strain of unicellular yeast was put under a selection pressure and started to display multicellular characteristics.

How is evolution 'seemingly' continuous when the real-world is seemingly so discontinuous and discrete?

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The real world might seem discontinuous to you, it doesn't to me.

Further to the uni-to-multi cellular thing, there are sponges that are multi-cellular, but force them through a sieve and they are reduced to single cells. But they will gradually reform and form a multi-cellular sponge again.

Tiberius said:

Further to the uni-to-multi cellular thing, there are sponges that are multi-cellular, but force them through a sieve and they are reduced to single cells. But they will gradually reform and form a multi-cellular sponge again.

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There is a whole range of cooperative behavior between cells, from bacteria that signal one another for coordinated behavior, through slime molds that usually live as single cells but that join together to form fruiting bodies, with some cells sacrificing themselves so that others can reproduce, through simple multicellular forms like these sponges, on to more complex metazoans that are purely multicellular (excepting gametes and zygotes). It's quite possible for multicellularity to develop by small steps.

leftrightleftrightleft said:

I am not a biologist. I fell asleep in high school biology.

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Firstly, what compels life to keep living? Why does a blood cell in my body (a component of life) continue its rounds carrying oxygen? What compels the bacteria currently infecting my throat to continue infecting? Quite simply, why don't living things just stop living?

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Some undoubtedly did. Unsurprisingly, those are no longer around.

On a mechanistic level, the "what" would be chemistry. The same basic physical laws that "compel" sodium and chlorine to form salt or water molecules to stick together as a liquid "compel" a virus or bacterium to latch onto a host cell. They operate in a much more complex system, but ultimately life is just chemistry. (The simple examples I gave involve ionic bonds and hydrogen bonds, respectively; two types of chemical bond that proteins often exploit both in folding themselves into a correct shape and in interacting with other molecules.)

Secondly, are viruses considered life?

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Depends on your definition of life

Thirdly, is there any theory out there as to how sexual reproduction evolved? Did some proto-sperm swimming in the primordial sea one day bump into a proto-egg and by some chemical miracle their DNA combined?

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Here's a nice one for how: The Origin of Sexual Reproduction - YouTube

Long story short: from ancient DNA repair mechanisms and eukaryotes' capability of cell fusion.

Fourth (and the longest; related to the third): Evolution is also described for simplicity sake as a basically continuous process. You'll hear scientists say that dinosaurs "evolved" into birds, as if this is a continuous, flowing process like pouring water into a cup. But isn't the real world actually terribly discontinuous?

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No, it's a lot more continuous than we are comfortable with Hence about a dozen conflicting definitions for "species", hence our inability to give you a straight answer when you ask whether viruses are alive, hence such in-between astronomical objects as brown dwarfs and dwarf planets, and so on.

As an example, take a multicellular organism. At some discrete point in history, there had to be only unicellular organisms. And then at some discrete point in history, there had to be multicellular organisms. Where is the continuous go-between that would allow one to get to the other without making a discrete "miraculous" jump?

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You don't necessarily have to avoid "miraculous" jumps. It can't be that difficult for a mutation to arise that simply causes cells not to separate after division. That is a potential one-step recipe for a multicellular organism that "reproduces" by breaking into pieces when it gets too large for its newfound cellular glue to hold it together.

(Now, to work out the ideal size and shape of a multicellular organism in a given environment would be slightly more complicated. It's still known to happen very quickly on occasion.)

Perhaps another example is better suited. At some point a prehistoric creature had to create milk from mammary glands. Was there not a discrete point in time where a creature first produced milk? How is there any go-between from a non-milk producing creature to a milk-producing creature?

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Perhaps it started with babies that got their daily salts by licking sweat off mum's belly