MadSci Network: Evolution

Re: Which evolved first (how, and how long, did it work without the others)?

Date: Thu Mar 23 16:00:42 2000
Posted By: Michael Onken, Admin, MadSci Network
Area of science: Evolution
ID: 953770951.Ev

Which evolved first (how, and how long, did it work without the others)?

The questions you've asked cover several evolutionary processes, which would take a lot more room than we have here to address thoroughly. I would strongly suggest reading through some Evolutionary Biology texts, if you want a more indepth discussion of any of these answers. That said, here goes:

a. The digestive system, the food to be digested, the appetite, the ability to find and eat the food, the digestive juices, or the body’s resistance to its own digestive juice (stomach, intestines, etc.)?

Metabolism is often cited as one of the prerequisites for life, so from the very beginnings of life (if not before) there has been a requirement (appetite) for certain chemicals (food) from which any organism gains materials and energy to grow and reproduce. In many microorganisms, some portion of the metabolism occurs outside the cell in order to break down complex chemicals into their usable components (digestion) for absorption by the cell. Later, some microorganisms adapted controlled motility, through flagella, cilia, and pseudopodia, which allowed them to move toward sources of food. If we fast forward to multicellular animals, we see the beginnings of the "digestive system" as early as the coelenterates (jellyfish, comb-jellies, etc.) which have a single mouth/anus which opens into a stomach. These evolved into "flatworms" which eventually separated their mouths and anuses to produce the linear digestive tract common among the protostomes and deuterostomes (including humans). Initially, digestion was accomplished entirely through enzymes, but internal digestion allows the organism to taylor the environment in which these enzymes function. The addition of acids, alkalis, and detergents to the digestive juices would have happened much later, and must have postdated the resistant lining which protects the digestive tract from the food as much as from the juices.

b. The drive to reproduce or the ability to reproduce?

Again, reproduction is a hallmark of life. Evolutionarily, anything that can reproduce itself will outcompete anything else that cannot reproduce itself. This has been shown to be true even at the molecular level in experiments studying the in vitro evolution of RNA enzymes (called ribozymes), so I don't think you can really separate the drive to reproduce from the ability to reproduce.

c. The lungs, the mucus lining to protect them, the throat, or the perfect mixture of gases to be breathed into the lungs?

The vertebrate lung evolved from modified pouches in the digestive tracts of early fish (in modern fish, these sacs have evolved into swim bladders). So, the throat and mucus lining (see part a.) were already present before animals with air-breathing lungs began to appear. I wouldn't call our atmosphere "the perfect mixture of gases": it is simply the mixture that was available when lungs evolved, and so our lungs have evolved to get as much use out of this mixture as possible.

d. DNA or RNA to carry the DNA message to cell parts?

According to the present theories, RNA came first and was used to code the genome as well as act as a template for protein synthesis. DNA was a later development and represented a more stable and more accessible medium for storing the genetic information.

e. The termite or the flagella in its intestines that actually digest the cellulose?

By "flagella" I'll assume you mean enteric bacteria, since flagella themselves are motile and not digestive structures. All modern terrestrial animals have bacteria in their digestive tracts that assist in the digestion of foods, so it is safe to guess that the ancestral species that gave rise to the termites were probably home to some form of enteric bacteria that could break down cellulose.

f. The plants or the insects that live on and pollinate the plants?

Fossil evidence suggests that plants began to populate the land long before animals: in fact, it wasn't until there were enough plants (food) to make it worthwhile that the first arthropods began to venture out of the water. The insects evolved from some of these first amphibious arthropods. Pollen-bearing plants don't appear in the fossil record until much, much later.

g. The bones, ligaments, tendons, blood supply, or muscles to move the bones?

Bones, ligaments, tendons, and muscle are all evolved from the same early connective tissues. Again looking at the coelenterates, the first mesodermal cells were contractile (that's how jellyfish swim) as well as producing a connective extracellular matrix. The division and differentiation into separate contractile cells (myocytes) and connective cells (fibroblasts) occured later. The vertebrate line is defined by a rigid rod (the notochord) which is composed of densely packed connective tissue (cartilage) layed down by specialized fibroblasts called chondrocytes. Bone evolved much later from the calcification of cartilage. The heart is common to all protostomes and deuterostomes, although the closed circulation is only seen in the deuterostomes. This appears to have evolved some time early in the chordates.

h. The nervous system, repair system, or hormone system?

I'm not sure which repair system you are refering to, so I won't hazzard a guess. The use of soluble chemicals (hormones) for communication goes back to single- celled organisms, so this clearly predates the nervous system which appeared quite early in the animals in the form of webs of sensory cells.

i. The immune system or the need for it?

There were bacteria and viruses on Earth long before the "higher" organisms. Since the cellular immune system is specific to the vertebrates (immunity, in general, is seen in both plants and animals), there was clearly parasitism and infection long before this particular strategy for dealing with it evolved.

Again, I would strongly suggest consulting a textbook that covers evolution to really understand these answers.


Current Queue | Current Queue for Evolution | Evolution archives

Try the links in the MadSci Library for more information on Evolution.

MadSci Home | Information | Search | Random Knowledge Generator | MadSci Archives | Mad Library | MAD Labs | MAD FAQs | Ask a ? | Join Us! | Help Support MadSci

MadSci Network,
© 1995-2000. All rights reserved.