Question: There are many examples where creatures rely on each other to survive which could not arise through evolution. Grass cannot survive without a certain fungus that helps it fix nitrogen from the atmosphere and the fungus can't survive without the grass. They must have appeared on earth at the same time. Same is true for the throat and mucus,and the digestive system and appetite. Which came first?
Response: Your first question is about symbiotic relationships, in which two sides need each other to survive because they work off of each other. Symbiotic relationships are all over the place… flowering plants, or “angiosperms”, need pollinators such as bugs to spread their pollen… And yet these insects need the pollen to create a food source. So one without the other will not work…
There is also a beautiful symbiotic system in the South American Cerrado involving mound building termites, leaf cutting ants, fungi, giant anteaters, the lobeira tree, and wolves. Ill number a few of the pieces of the system.
(1) The mane wolf in the Cerrado though hunts small animals, it’s diet consists mainly of the fruit from the lobeira tree. (2) The lobeira tree in turn depends on the wolf for its continued existence as the seeds will not germinate until eaten and passed through its digestive system.
(3) The Cerrado’s giant anteater produces one baby in which she will carry on her back for up to a year, and both of them eat thousands of termites per day. The anteater, though consuming thousands of termites, do minimal damage to a termite mound as it feeds for only ONE minute before moving on to the next. (4) This allows the termites to quickly repair the damage and it does not wipe out the anteaters food source.
(5) Once a year during the rainy season, the myriad of termite mounds in the Cerrado glow bright green because larva of the headlight beetles have been laid within the mud structure. (6) Every year when the termites begin their mating flight, the light produced by the larva attracts the flying termites, ending in the termite’s death and the beetle’s meal.
(7) The local ants come out and pick the seeds out of the wolf's fecal matter and transport them deep into the ground where they feed a species of fungus which breaks down the seeds. (8) Sometimes the seeds arent broken down but actually germinate in the ground, which are under termite mounds, and then spring up to growing the tree.
So 1-8 all together: These trees would not be able to survive without the nutrients the ants bring into the ground, but the ants would not have the seeds without the wolf eating the lobeira fruit… And the beetles and anteaters would not have a food source without the termites… And the termites would never survive without the anteater’s internal clock of spending only 1 minute per mound…There are symbiotic relationships all around us, from the bacteria in our gut needing our digestive tracts to survive, and us needing the bacteria to digest certain foods…to the examples you gave a certain grasses needing a fungus to survive… They are all around us.
Trees need nutrients found deep within the soil. They could evolve more efficient root systems that would allow them to extract those nutrients themselves but this can take a lot of time and might not happen at all. It just so happens that fungi already have this ability, so when two species find themselves in close proximity, it is much faster to evolve a way to incorporate another organism’s ability than to reinvent it itself…Like I said earlier, insect evolution is closely related to the evolution of flowering plants. The symbiotic relationship is quite obvious considering that about two thirds of flowering plants are insect pollinated.
So how could separate species evolve traits that happened to fit together so well, and cannot function without each other?
Well like everything in evolution, natural selection is the answer. In a population of organisms, some will have traits that are more advantageous to successful reproduction than others, and over many generations the population will inherit the successful traits. Traits that allow creatures to take advantage of another life form in it’s environment will be just as successful as the traits that allow it to escape or eat them. So when these traits first appear they will be optional, with some individuals taking advantage of another and some not, but if taking advantage of another organism becomes advantageous…natural selection will begin to select the organisms that inherited traits that allow them to do this. Eventually the symbiosis can become a sole source of food, shelter, enzymes, or anything else that these relationships derived from one another.
This question is also linked to the idea of the irreducible complexity, in which if you take a part away from something, it can no longer function. This is a popular term but it is not a problem for evolution. There is a great example of how complexity can evolve, and I will try to explain it as best I can.
Example: Imagine a unicellular organism in which has two specific entrances for two specific chemicals…I will call them entrance (A) and (B), and chemical (a) and (b). At first, the organism must intake chemicals (a) and (b), through specific entrances (A) and (B) because it needs these chemicals to survive. When the organism replicates, a mutation occurs that actually allows the organism to intake chemical (a) and actually break it down into chemical (b)... At this time the organism still has two entrances (A) and (B) but with the new ability to break down the chemical (a) into (b)s, it no longer needs the second entrance (B) . This trait is passed on as it is slightly more advantageous for the organism, and throughout the generations if a mutation occurs that changes the structure, the size, or anything about entrance (B) , it will not matter because it can create chemicals (b) inside it’s walls. Eventually this species will build up mutations that cause entrance (B) to disappear... But it still retains the ability to create chemical (b) FROM (a).
Imagine this organism lives today but we are studying its ability to make chemical (b) from (a)… But someone comes along and says “this system is irreducibly complex because without the ability to create chemical (b), the organism would die”. But this is not true because in it’s past it had a different ability that allowed it to intake both chemicals and it evolved the ability to make chemical (b) from a later on…thus allowing it the option to get rid of that second (B) entrance and only keeping the (A) entrance.
This is pretty much what happens with co-evolution and symbiotic relationships. It is always in incremental steps, and when something appears that is slightly advantageous, and allows for the loss of something else (entrance (B) ), the need to rely on each other begins to build up as functions are lost because they use each other more and more. The end result is something that looks like it cannot survive without all of the parts, but it is because they evolved using each other and lost several of its previous abilities in the process.
So in the example you gave of the grass needing the fungus, and the fungus needing the grass, it would have arose in the same fashion. In the ancestral versions of this grass and fungi, each would have been able to survive alone. Eventually due to cohabitation, once one of them discovers a way through mutation to use less energy itself by relying on the other and… a relationship occurs. It is not required in the beginning, but the energy saved by relying on another organism is tremendous, and thus is quite advantageous…as well as common. Once the relationship is established, previous functions will begin to disappear as they are no longer needed because it is gained through the use of another organism. So for the grass and fungi, they began to use each other in the beginning and when a mutation developed that hurt a previous function it did not matter…and was passed on. And the end result is fungi and grass that are reliant on each other because they let each other lose their previous abilities as long as they continued to provide for one another.
So, neither one of the current forms came first as what you see today did not exist in the past. The ancestral forms would not have been so dependent on one another, and it is not after millions of years that the acquired their current forms of being dependent.