Evolution - How did different systems of organs come into being?

There are three basic modes by which food can be absorbed: from an external surface, an internal pouch, or an internal canal. There are numerous animals that use the canal configuration and it develops from a fairly similar mechanism. Gastrulation is the process by which a ball of embryonic cells forms into a hollow sphere with a pore. The formation of three distinct layers of tissue occurs at this point: the outer layer, ectoderm, which produces the outer surface (e.g. skin) and nerves; the middle layer, mesoderm, which forms muscles and other connective tissues; and the inner surface, endoderm, which forms the surface of the digestive pouch, or, if a second pore develops, tract. All three are present in modern organisms.

Once the patters which have formed a simple tract are present, the tract may undergo slight modifications that specialize it over its length. The segmented earthworm offers insight. A bulge in the inner tube produces a storage area, the crop. A narrowing transit. Different segments can have specialized cells that help break down material of extract nutrients.

There is information in the environment; for example, sand dunes record wind direction, speed, and variability in their shape, but this is only tangentially related. The idea of evolution is simple. Organisms aren't absolutely identical. There are small changes. If one organism has a slight variation that offers an advantage in extracting energy from food, it is more likely to have the energy to reproduce and pass on that advantage. Over time these little advantages accrue. A cluster of cells that offer advantage become a larger cluster, and over enough generations off added advantage and changes that result in increased size, become an organ.

Note that it can seem quite overwhelming when one looks at multicellular organisms like ourselves without first looking at some simpler organisms and asking how they dealt with the problem of getting enough nutrients.

This is a crucial point. When we look at simpler organisms we see a fundamental difference in how we manage this energy problem. In single celled organisms they can quite easily allow molecules to pass through their cell walls/membrane and receive adequate energy. This is the simplest type of digestion/metabolism wherein the cell uptakes nutrients from it's environment and outputs molecules which are non-synonymous to the molecules which entered. Other more complex organisms like fungi (which are still pretty simple), excrete certain enzymes into the soil (extracellular environment). The enzymes then breakdown specific targets into products which the fungus can then absorb and use for energy (link). More complex organisms like the jellyfish have a rudimentary digestive system where water, nutrients and oxygen enter the bell of the organism due to its swimming motion. This cavity is the same cavity which expels the waste products. A subtle change in a jellyfish like ancestors' genome could have simply changed the shape of the bell to allow water and nutrients to collect in a small cavity. In this cavity because the organism is releasing Co2 the PH of the water and nutrients in the cavity is decreased/more acidic (link). This simple coincidence could have been a simple starting point for a more complex digestive system. Natural selection could have favoured individuals which had a deeper cavity or could sweep food into it more efficiently as they swim, or have enzymes which function optimally in the new PH environment and possibly exploit some additional metabolites in the process(link) (projections could help grab food or push it in towards the bell- look at the video to see precisely this). Then there are things like earthworms which have a one-way digestive system. Like ours - in one way and out the other. Look at this pdf for more information - if you want the a proper explanation you need to look at genetic evidence, fossil evidence, phylogenomic analysis etc of which there is plenty.

The digestive system works in sync with the circulatory system. After all; what use is the digestive system if the blood cannot absorb the nutrients and transport them to all areas of the body. We should see a relationship here. When we look at things like jellyfish etc they do not have circulatory systems although they have this simple digestive system. When we go up into things like worms; some have a very simple circulatory system. It seems clear that to be a multicellular organism then we need to be able to efficiently extract nutrients from our environment. It seems clear that in multicellular organisms with digestive systems like ours are warm blooded. Warm blood speeds up reactions. Raster reactions allow quicker metabolism, quicker metabolism allows more time spend doing things other than eating, like finding a mate, raising offspring etc It seems strange not to expect to find ourselves complex when we possess all the other characteristics that we do. When we look in the digestive system we can see how the blood vessels innervate the small intestine and carry nutrients to the body; there are little projections from the surface of the intestinal epithelium known as micro villi, they contain the bloodvessels which absorb nutrients. It seems logical that natural selection would promote these projections as they allow more cell surface area to absorb nutrients. This could be selected for quite easily- for example if a mutation occurs which causes a gene to be overactive in intestinal epithelial cells, then if this gene codes for something like microtubules it should be not surprising to find that cell with large projections (filopodia link). Not everything just pops into form already made. It occurs over many thousands of generations and make small gradual adjustments.

I would even argue that it's the genetic evidence which is the most illuminating to the process of evolution in our life history. Either all of this occurred by observable, explainable natural processes or a supernatural being did it. It just doesn't need any kind of supernatural explanation, the problem is, is that people don't try to learn enough about it. When you see enough of the evidence and think about it honestly, I fail to see how an individual could argue with at least the fundamental premise of evolution. Unfortunately people see it as hard and take shelter in ignorance.

EDIT: read up on the development of digestive systems an different organisms. Try to jot the names of key genes, then compare the genes between different organisms. We see that alot of these genes have very similar sequences and provide the same general function but maybe in a new context. This is molecular evolution.

I really believe that God has almighty power over how we are formed and what we are formed by. The scientific answer would be that everything starts as a cell in the very beginning. We start as an embroy and from there we grow as the cell performs mitosis. But God has almighty power.