Lunnor

How did herbivores and carnivores come about? I would assume it started out as all autotrophes, but somewhere some "thing" started eating another "thing", so...how did it all start? Please link to whatever resources if possible. Okay, just don't link to the bible's genesis.

Lunnor

Automaton

You mean, how did life start "eating" other life? My guess is something like: Mutual aid -> Symbiosis -> Parasitism -> Predation.

Doubting Didymus

I don't think that autotrophs are the standard at all. I think that organisms had to become quite advanced before they could be self sustaining (the most common example being organisms posessing chloroplasts).

I suspect that heterotrophy is as old or older that autotrophy in a sense. The very earliest self replicating molecules would have needed rescources to build new molecules, and I also imagine that, past a certain point of evolutionary development, the best source for those rescources would stop being natural sources and start being other self replicators. (which would not really have been 'eaten', but certainly could be scavinged as a resource if broken down. So I think heterotrophy would be as old as the hills.

Automaton suggests that predation could have evolved from parasitism, which is possible, but I would rather point the finger at scavengers. That is, predation evolved from scavanging organisms, from which a smooth gradual slope would exist to take them up to eating freshly dead/ just dying/ live prey.

lpetrich

Here's <a href="http://www.ucmp.berkeley.edu" target="_blank">a good site that discusses evolution</a>. And here is the more-technical <a href="http://tolweb.org/tree/phylogeny.html" target="_blank">Tree of Life</a> site.

The most likely initial sort of metabolism is, indeed, autotrophy -- manufacture of all biomolecules from inorganic ones in the fashion of plants. This, of course, takes energy, and the most likely initial source of energy was chemical reactions like H2 + S -&gt; H2S (hydrogen + sulfur -&gt; hydrogen sulfide). Some of these organisms learned the trick of capturing energy deposited by incoming photons, becoming photosynthesizers, and about 2.3 billion years ago, some of these worked out how to crack water, becoming cyanobacteria (blue-green "algae"), and the ancestors of the chloroplasts of plants.

As to what the first predators were, it's a matter of definition.

Bacteria have been around for 3.5+ billion years, and some of them may be described as predatory. Bacteria cannot swallow anything much bigger than a sugar molecule, a limitation that some bacteria get around by releasing digestive enzymes into their environment. These enzymes snip up the big molecules around them into bite-size chunks that the bacteria can easily transport through their cell walls.

Some bacteria, like Bdellovibrio, actually live inside the bacteria that they are currently eating; they suck the host bacterium dry, as it were.

How long ago these capabilities emerged I'm not willing to speculate on, but progress in molecular-level genetics may make it possible to answer that question some day.

Going from external to internal digestion helps avoid letting digestive enzyme go to waste, but it requires the ability to swallow prey much larger than a sugar molecule.

And that capability emerged in some early eukaryotic organism, an early protist. perhaps about 1.5 billion years ago (here's <a href="http://www.biomedcentral.com/1471-2148/1/4" target="_blank">an excellent paper</a> on their emergence). Eukaryotic cells have a "cytoskeleton" formed from fibers originally used for helping the cell divide, and this cytoskeleton can be used to change the cell's shape.

So if that protist feels a bacterium touching it, it can pull in that part of its membrane, eventually creating a bubble around that bacterium, a process called "phagocytosis". That protist can then secrete digestive enzymes into that bubble, and soak up that digested bacterium.

Many of their descendants have continued this tradition of bacterium eating, but some, notably the fungi, have reverted to a bacteriumlike external digestion. And some of their multicelled descendants have invented organism-scale internal digestion, becoming the ancestors of most of the animal kingdom.

One side effect of eating bacteria is that some of them may not be digested, but may instead take residence inside of one's flesh, in some cases, providing useful services ("endosymbiosis"). And there is evidence that this has happened to some early protists.

One such bacterium ended up doing a final stage of digestion for its host, combining simple food molecules with oxygen, releasing lots and lots of energy. This bacterium became the mitochondria, which still have their own genetic apparatus, despite losing many of their genes to the nucleus.

Another one was a cyanobacterium; it became the ancestor of the chloroplasts of algae and plants, producing food molecules for the host cells.

This process can sometimes be repeated, with a protist "eating" a photosynthetic protist, producing a multimembrane chloroplast with a vestigial nucleus.

[ October 20, 2002: Message edited by: lpetrich ]</p>

Doubting Didymus

Round of applause for Lpetrich.

lpetrich

As to heterotrophy being ancestral, that is certainly necessary for the ultimate ancestor -- complete biosynthesis would not have emerged all at once. A very early organism would have to have lived off the Primordial Soup that it had formed from.

But all existing Earth life has had some Last Universal Common Ancestor (LUCA), also called the progenote or the cenancestor. This organism or community of organisms had had a shared RNA-&gt;protein genetic code, DNA as the master-copy molecule, etc. So what kind of metabolism had it had?

I've checked some discussions of LUCA, like <a href="http://www-archbac.u-psud.fr/Meetings/LesTreilles/LesTreilles_e.html" target="_blank">the Les Treilles conference</a>, and <a href="http://aca.mq.edu.au/workshop.html" target="_blank">this astrobiology workshop</a>. The latter mentioned plans to create "Archean Park" LUCA proteins by reconstructing their sequences and checking on their properties.

There was not much on its metabolism, but one interesting controversy was whether it and its RNA-world predecessor had really been thermophiles. Many of the earlier branches in Bacteria and Archaea had been thermophiles, which suggests a thermophilic ancestor that liked to live in hot springs at temperatures of 80-100 C. Some counterarguments are that the early branching is an artifact of adaptation to high temperatures, and that a thermophilic LUCA could have been the descendant of some lower-temperature-preferring organisms that got selected because it could survive a big meteorite strike by living in an underwater hot spring.

It was difficult for me to discover what biosynthesis could be reconstructed for the LUCA, but a reasonable guess is that it could manufacture most of its more complicated molecules, because they would have been rarer in the Primordial Soup. So if the LUCA had been heterotrophic, it would have been dependent on only relatively simple organic molecules.

The LUCA's energy source was most likely chemolithotrophy -- eating inorganic materials like H2, CO2 and sulfur compounds, with perhaps some simple organic compounds from what was left of the Primordial Soup. Photosynthesis only emerged in one of its descendant lineages, the Bacteria; it is absent from the oldest branches of Bacteria and from most of Archaea.

Interestingly, a major subgroup of Archaea, the methanogens, is almost exclusively autotrophic, subsisting on

CO2 + 4H2 -&gt; CH4 + 2H2O

with some methanogens capable of using CO2 substitutes like methanol and acetic acid.

And this is despite numerous opportunities for eating organic molecules in many of their present-day habitats.

[ October 21, 2002: Message edited by: lpetrich ]</p>

lpetrich

I've found <a href="http://caspar.bgsu.edu/~courses/evolution/xLectures/Lect02_04.html" target="_blank">this nice page on the LUCA</a>.

According to that page, the LUCA likely had had enzymes for carbon fixation, meaning that it had likely been autotrophic. If it was heterotrophic (dependent on eating organic molecules), it would not have had much need for carbon fixation, capturing CO2 and using its carbon in organic molecules.

It also likely had lots of biosynthesis enzymes -- amino acids, nucleotides, coenzymes, etc.

Meaning that the LUCA had not been adapted for eating other organisms.

I'm being a bit tentative here because I'm not sure what the family trees of these enzymes look like. If they have a split between Bacteria and Archaea like what is obtained from ribosomal RNA, then one can be confident that the LUCA had had them. If, however, they look like they originated in Bacteria and spread to Archaea or vice versa, then the LUCA may not have had those enzymes.

abe smith

LYNN MARGULiES, bright person, offered vee plausible possibility: that whaddyecall = eu-- I forget the words! = nucleated cells, as opposed to NONnucleated cells, came into being when one(kind of primitive) cell "ate" = engulfed another kind of cell &gt;&gt;&gt; (maybe a Euglena = w/ chlorophyll); and/but did not digest it ==(not) broke it down; but let it continue to do its ordinary behaviour INSIDE the "eater" cell= thus giving rise to photosynthetic cells, w/ chloroplastids. Check out Margulies's books.
Oh: EUKARYOTES = nucleated = "sophisticated" cells. PROTOKARYOTES = early, nonnucleated cells.`Protokaryotes reproduce by fissing = no chromosomes = no mitosis. Eukaryotes reproduce by mitotic (chromosomal) division, &gt;&gt;&gt;&gt;
giving rise to (later) sexual /meiosis.
Fascinating.

lpetrich

First off, I hope that I have not scared off Lunnor.

And abe smith's comments seem rather confused. But I haven't found any sites with really good diagrams illustrating endosymbiosis.

But I've found <a href="http://protist.i.hosei.ac.jp/Protist_menuE.html" target="_blank">this nice site with lots of protist pictures</a> -- which shows some examples of present-day endosymbiosis, algae infesting some nonphotosynthetic protists.

Oolon Colluphid

Just wondering where bacteriophages fit into this. Are they (or viruses in general) highly derived, 'modern', or do they represent something really ancient? I suspect, lifestyle-wise at least, the latter: that once there became a shortage of supplies (nucleotides or whatever), then those replicators that could pinch bits from the 'bodies' of others would have a survival advantage over those that couldn't.

If you view the predator-parasite lifestyle as simply stealing resources from others, I'd have thought that it would have started long before things as complex as bacteria came along.

Oolon