The three theories
Savannah theory is that the early humanoid primates developed bipedalism due to the deteriorating forestation to be able to travel on the ground with more efficiency. Having a bipedal gait would infer many advantages over other competing primates for food and therefore survival. Such as the ability to travel longer distances without overheating, longer lines of sight, greater speed across the terrain and much more efficient energy conservation. All of these advantages would have been hugely important for survival in a time where food was scares due to the cold winds killing the forest and increasing the grass lands.
The savannah theory does not include how every much on how we started to develop these evolutionary changes
Aquatic ape theory
This theory states that before early humanoid apes arose on the savannah plains they spent some time in water. This is due to the peculiar layer of fat under our skin, much like sea dwelling mammals rather than other apes. The theory suggests that we lost the hair from our bodies through this process as well but that is questionable as water dwelling mammals like otters still have fur on their bodies. What led early man to the water was a lack of food in a dying forest and an ever growing competition for food.
The presence of the mammalian dive reflex links us to water also. This reflex slows our heart rate and allows us humans to free dive to great depths making us excellent fishers and very able at collecting shell fish from as deep as 20 metres or so.
Humans, according to this theory would have first learned bipedalism in the water by rooting through the shallows while keeping their head out of the water before venturing deeper to find more rewards. This rooting in the shallows forced early humans to stand on two feet, supported by the water to help them.
Aquatic ape theory fits really well. Seafood helps to create and improve the human brain, also some other anecdotal evidence such as our ability to control our breathing patterns better than most other land mammals, more like aquatic mammals. This improved breath control gives us the ability to speak in a more complex way. Some of these points can be argued as highly anecdotal but it is hard to find another reason for these traits based on just growing from the savannah.
Coastal theory is very much a similar theory to the aquatic ape theory but it focuses more on how the seafood diet would affect the evolutionary process from ape to human and especially at the speed it seemed to happen.
When we look at the fossil record we see an overall lack of bones. What I mean by that is we only have 10 fossils that show any transition from the ape to human foot, which is not much to go on. In fact we have a good number of fossils of our chimp ancestors and more fully formed ancestors but we have very few fossils of the transitional ancestors, with little fossil evidence found on the coast as well. Coastal theory would have you believe this was due to rapid evolution of the human ancestral species due to epigenetic conditions at the time. This would leave a lack of fossils for us to find.
Considering the four largest glacial periods happened in line with human evolution, one being precisely when man showed up on the fossil record around 2 million years ago, it would make sense that certain groups of apes would become shut off from others. This would drive them to search out new territory and food sources as the cold destroyed the forest in favor of flat grass land. As the forest died away and the weather became either wet or icy, climbing trees would have become dangerous. So the option of a terrestrial diet (land based) would have been needed. We can see today, some apes with more human-like feet who live in wetter conditions and spend more time on the ground. So the need to walk more suggests the need to be upright and this is proven that we walked before we had big brains through fossils that have been found called Lucy & Ardi who were bipedal but had not yet developed big brains. Coastal theory, like many theories, agree that when the forest started to disappear early humans developed bipedalism and looked further afield for food, however the rest of the details are what are still contested.
So, during this time of mass geologic and environmental change many creatures became extinct and food was scarce but in the coastal regions you had very plentiful shell fish sources, although this would not of been the apes first choice, when faced with starvation any food is better than none.
Something we can learn from genetics is the existence of jumping genes that us humans and apes have. This could have been the deciding factor in our evolution. Jumping genes were discovered by Barbra McClintock in 1951 (which 30 years later she received a noble prize for) and they allow us to store and assimilate on their y chromosome. This now would be considered junk DNA or non-coding DNA that can either be switched on or off due to epigenetic stress. Coastal theory has shown shellfish to be our most likely source of food and this shellfish would have been likely loaded with bacteria and viruses.
One in particular would have been the Vibro species, which also is a water born pathogen (transmitted through water). These bacteria would have likely killed a few of the apes but the ones that survived would be now able to adapt to their new diet and it turns out this diet could have created some amazing changes. The vibro bacteria also secretes Zonulin, which causes leaky gut, which would allow us to assimilate more of these bacteria from our new found diet. This change can answer for our extreme environmental adaptability and large ability to handle multiple food sources as humans. This also means we have a leaky gut by design and not as a result of some major illness. The question would be is your environment selecting to open the barrier or not?
Today the human genome is considered to be 97-98% non-coding DNA, that means only 2-3% of our DNA is in charge of structural protein transcription. It’s argued that another 50-60% or so could be non-structural based but that would still leave around 40% of our DNA as dormant. 10% of the human DNA structure today also is assumed to have viral origins and is mostly stored in the Y chromosome, which controls brain growth.
Next we look at the parts needed to create a human brain, which weighs 3lb and is predominantly made of fats. Many people believe it would have been impossible for the human brain to evolve from a primate one on skeletal meat or vegetables alone. They think seafood would have been our staple diet in this situation, just for the sheer amount of DHA you would need alone. Humans have very little ability to convert plant fats ALA into EPA or DHA. DHA is a constant building block for the brain and nervous system.
The final point that coastal theory brings to the table is an answer to the question ‘if this all happened, where are all the bones?’ The idea is that the bones would have been dissolved by the sea water. This is possible scientifically but is still questionable. One other thing we need to consider is that the fossil record for transitional apes is incredibly poor, so this theory of rapid evolution on the coast could hold water just as much as other theories, if not more so.