27 Which evolutionary innovation has been the most influential, and why? Grace Mortiz

Which evolutionary innovation has been the most influential, and why?

Grace Mortiz Upper Sixth

Throughout the history of animal life, environmental and ecological shifts have resulted in both small and large scale evolutionary diversifications. Examples include the invasion of land around 400 million years ago, giving way to new habitats and niches for many organisms (Erwin 2015). Mass extinctions, namely the one at the end of the Permian period which wiped out 95% of earth’s species, have also created new evolutionary opportunities (Lumen Learning n.d.). These events have been the origins of evolutionary innovations, which are defined as ‘traits that produce something novel compared to before’ (Dall’Olio 2012). In this article the evolution of warmbloodedness, the eye and the mitochondria will be discussed, in order to conclude which of these innovations may have been the most influential.

In today’s world, the only warm-blooded animals, otherwise known as endotherms, are mammals and birds. The origins of warm-bloodedness is a controversial issue because anatomical features that were thought to be linked to warm bloodedness have also been found in cold-blooded reptiles. These features include turbinates, which are bones inside the nose or snout, which increase the distance that air has to travel to get into the body. This allows it to warm up on the way in. What we do know is that the development of a warm-blooded metabolism allowed ancestors of mammals and birds to survive the giant mass extinction at the end of the Permian around 252 million years ago (Rey n.d.). internally (Smithsonian Tropical Research Institute 2019). Endotherms can maintain a constant high body temperature internally, which allows the immune system to deal with pathogens more effectively. Logan, a Tupper Fellow at the Smithsonian Tropical Research Institute (STRI), suggests that “by keeping their bodies warm at nearly all times, mammals and birds effectively prime their immune systems to withstand virulent pathogens, and this may be part of the reason the extremely costly strategy of endothermy evolved in the first place.” Perhaps therefore, warm-bloodedness holds critical advantages over ectothermy, making it a particularly influential innovation. A recent study that looked at the ratios of Endotherms are not restricted by thermal oxygen isotopes in fossils has suggested variation and can enter new habitats and that ancestors of mammals may have ac- niches. However ectotherms may be sensiquired warm-bloodedness tive to environmental flucbetween 259-252 million years ago, earlier than Endotherms are tuations, and in searching for warm spots to initiate previously thought. This not restricted ‘fever’, they may struggle in turn suggests that climate may have been the selection pressure that by thermal variation and to find food, to mate, and may be more exposed to predators. (Smithsonian triggered the evolution can enter new Tropical Research Instiof warm-bloodedness (Rey n.d.). There are other theories as to why warmhabitats and niches. tute 2019). Schwab (2018) describes bloodedness arose in our the existence of eyes as ancestors - for example, as a result of path- “so basic to our profession that we do not ogens. In order to achieve ‘fever’ like high consider how and why vision appeared or body temperatures that boosts the effective- evolved on earth at all”. He believes that ness of some immune cells, cold-blooded “sight is an evolutionary gift” but it was animals, or ‘ectotherms’ would have had to not an “ineluctable” one. Through the sit in the sun for extended periods, as they process of evolution by natural selection, cannot maintain a high body temperature different types of ‘eyes’ have emerged over

Smithsonian Tropical Research Institute

Figure 1

time. It is estimated that the first animals with some kind of simple ‘eye’ structure lived about 550 million years ago. Using the less complex light sensitive structures found in living organisms today, scientists have suggested scenarios by which the first eye like structure may have come about, perhaps starting out as a simple, pigmented, light sensitive spot on the skin. This light sensitive spot may have given the organism some kind of small survival advantage, for example, allowing it to evade a predator. A random mutation at some point in time may have led to a depression in the light sensitive patch, sharpening the ‘vision’ further. At the same time, the rim around the pit may have narrowed, allowing light to enter through a small hole - like a pinhole camera. Every change that the ‘eye’ underwent would have had to give some kind of survival advantage in order to be passed on. Eventually, the light sensitive cells evolved into the retina, the layer of cells and pigment which is situated in the back of the eye. Over time a lens formed at the front of the eye. Interestingly, eyes corresponding to each and every one of these stages have been discovered in living species today (Nilsson n.d.). This fact supports this particular scientific theory about how complex eyes, like that of a human, may have evolved.

Clearly, the evolution of the eye has been a massively influential innovation. It gave our ancestors and existing species today the ability to see what was around them. This has given them a survival advantage in terms of allowing them to more easily forage for food, evade predators find prey and move into new niches. It should be noted however that organisms with poorer vision make up for this with other evolutionary ‘gifts’, an example being bats, which use echolocation in order to find their prey. Perhaps one of the earliest, and most influential innovations in our evolutionary history came about when only 2 domains existed, the prokaryotes Bacteria and Archaea. Somewhere between 2.7 and 3.5 billion years ago (Virtual Fossil Museum n.d.), an Archaea engulfed a Bacterial cell, perhaps attempting to eat it. Somehow this bacterial cell survived and was able to replicate inside the archaea. Equally, the Bacterial cell was beneficial in some way to the Archaea. Eventually the Bacterial cell became what we know today as the mitochondria - often referred to as ‘The powerhouse of the cell’, and a new Domain - the Eukaryotes, was born (see figure 1). This is known as the Endosymbiotic theory. The mitochondria is an extremely important organelle essential to eukaryotic life because it is the site of cellular respiration, the process by which oxygen and glucose are converted into an energy storage molecule, ATP. This ATP is vital for metabolic reactions, cell growth and division, and therefore the survival of a whole domain of organisms (MitoQ 2017).

In conclusion, the evolution of warmbloodedness, while costly in terms of energy loss, is highly influential in giving a survival advantage to mammals in birds in terms of opening up new niches where environmental conditions are different. It also may be highly beneficial in terms of the immune system. However, a high proportion of the animal kingdom is coldblooded, so perhaps ectotherms make up for it with minimal energy loss. The eye is certainly a gift of evolution, opening up new opportunities that organisms did not have before. But, as Schwab (2018) noted, the gift of sight was not ‘ineluctable.’ Without good eyesight, organisms have evolved other methods of finding prey etc, like echolocation, which is likely just as effective as good eyesight in its own way. It cannot be ignored that the evolution of the mitochondria has had a huge effect on the species alive today, allowing the survival of all multicellular organisms on earth, as well as much of the unicellular life, like yeast. For this reason, it should be considered one of the most influential innovations in evolutionary history.