(Un)expected Visitors 2: Curiosity Gene
By: Paul Rommel Elvira
Tags: Evolution, Neoteny, Humans, Zombie Snail, Curiosity Gene

Curiosity Gene

“The important thing is not to stop questioning. Curiosity has its own reason for existence. One cannot help but be in awe when he contemplates the mysteries of eternity, of life, of the marvelous structure of reality. It is enough if one tries merely to comprehend a little of this mystery each day.”- Albert Einstein

There is, indeed, a “curiosity gene.” That gene is called DRD4, which stands for dopamine receptor D4. It was identified by Max Planck Institute researchers in 2007. DRD4 is responsible for building receptors for the neurotransmitter, dopamine, in the brain [1]. The existence of a gene for curiosity indicates that this behavior is an adaptive trait. It is interesting to note, however, that DRD4 was not discovered first in humans but in a passerine bird, Parus major. This gene was observed to be involved in a strong exploratory behavior in the songbird, the great tit.

In humans, a variant of DRD4 was found to be associated with a risk-taking phenotype. Extraversion and novelty seeking behavior among skiers and snowboarders was reported to be linked to the -521 C/T variant of the curiosity gene [2]. The authors noted, though, that this gene variant was not associated with impulsive sensation seeking.

One of the most relevant books discussing the role of curiosity in human evolution is Alexandros Kourt’s The Curiosity Gene [3]. According to this book, curiosity is responsible for the survival of humans as well as for our evolutionary advancement to become the most intelligent creature on Earth. Kourt claims that curiosity is the missing link in the story of evolution of humans. Intense curiosity is a unique characteristic of humans, setting us apart from other animals. The origin of human curiosity can be traced back to 2.3 mya, finding its way into the human genome and driving our success as a species. This book is an interesting look at the importance of curiosity in human’s evolutionary history. However, Kourt stopped short at the suggestion that curiosity holds the key to our happiness and personal fulfillment — that’s it. Curiosity is a means to an end which is survival. But, what happens if you stretch the usefulness of curiosity beyond such primal application? Can it point us to an answer regarding the question, “where is evolution headed?”

Curiosity appears to have a more important purpose if we view evolution in the eyes of alien civilizations that are studying how organisms emerge on Earth. In this scenario, curiosity is a tool to help organisms to rise above everything else to look for their real origin or their ‘creator.’ Humans are not just content with satisfying our curiosity, curiosity drives humans incessantly towards wherever it leads us.

Discovering what lies beyond Earth might not be beneficial to us in the short term but why do we still do it? If other members of another intelligent species of animals see us investing invaluable resources to explore outer space, they might think that we are crazy because no apparent benefit will come out of such an exercise. They might even laugh — if they are able to laugh — at NASA scientists for naming their Mars exploration rover Curiosity Rover! They would likely ask, “Can curiosity feed you or make your species better at reproduction?” Of course, we would answer, “No, but its results might — in the future.”

According to Richard W. Byrne from the Centre for Social Learning & Cognitive Evolution and Scottish Primate Research Group, information gathering, which is basically what animals do when we are satisfying curiosity, is worth the effort even if there is no clear reward for doing it — unless it is unreasonably costly or risky [4]. For humans, this does not appear to make sense because, as we know, (aside from the fact that building rockets is literally very expensive!) venturing into space that lacks oxygen and gravity — not to mention teeming with radiation — is certainly costly and risky.

Evolutionary biologists suggest that human curiosity is associated with our trait called neoteny which means ‘retention of juvenile characteristics.’ According to this concept, the human species is more child-like compared to other mammals and this is demonstrated by our being relatively hairless and having a big brain relative to our body size. Therefore — proponents of neoteny claim — the playfulness and curiosity that humans display throughout our lifetimes is a behavioral manifestation of neoteny. Also, neoteny is considered as an evolutionary shortcut resulting in humans being more juvenile than non-human primates, making us weaker than our relatives. However, it also made us highly curious organisms with great capacity for learning [5].

If we accept neoteny as the reason behind the extreme curiousness of humans, then we can say that humans are just literally wandering about on this planet with no apparent direction or purpose. And we can stop there because human evolution has reached an obvious end to its story. But we can also say that life has been designed that way — to continuously search for something interesting, whether on this planet or somewhere else.

However, if we consider humans as having more special or advanced cognitive abilities compared to other animals such that we can override basic evolutionary laws, then curiosity takes a different context. It would then seem that curiosity is a proof that humans are looking for something else, something that could explain or complete our existence. It is as if we have another life cycle and that part of the cycle is not yet found or completed by humans on Earth (for now). So we keep on searching for that missing piece. How else can you explain the purpose of extreme curiosity in the big evolutionary picture?

This human’s search for an apparent next stage reminds us of the story of zombie snails. Zombie snails are called so because they behave like mindless snails, attracting predators instead of hiding from them. This behavior is due to an infection by Leucochloridium paradoxum or the green-banded broodsac. L. paradoxum is a parasitic flatworm that typically infects gastropods, like the land snail Succinea putris, as an intermediate host [6]. The parasite starts as a larva inside the host and grows into a sporocyst in the snail’s digestive system. Sporocysts then develop into broodsacs containing numerous cercariae. These broodsacs travel and invade the snail’s eye stalk. This turns the eyestalk into an eye-catching structure — especially for predators — due to its colorful, pulsating and swollen appearance resembling that of a wriggling grub or caterpillar. The broodsacs’ invasion of the eyestalk causes the eye to lose its ability to perceive light intensity so the snail tends to go to open or elevated areas where there is more sunlight since the snail most likely finds all the other places much darker than they really are due to the broodsacs obscuring its eyes. Staying in open or elevated areas exposes the snail to higher chances of being eaten by birds. Inside the digestive system of the bird, cercariae become adult diastomes that are capable of reproduction. Diastomes then lay eggs that are excreted by the bird along with its droppings. The cycle is then repeated when snails consume the droppings containing the L. paradoxum eggs.

This analogy does not imply that humans are zombies that are fully unaware of what is going on around them. This is just to illustrate that there could be a deeper process that could be operating inside us. Obviously, we appear to be healthy and our minds are not clouded by any overt foreign body like the broodsacs but is nonetheless causing us to behave in weird ways (such as wandering aimlessly in the name of curiosity). But what if what is inside us do not appear as foreign? We do not appear to have strange organs or body parts that can control our decisions or actions. What we have, though, is bits of foreign DNA in our very own genome.

Bacterial genes have been found to be introduced into human somatic cells, cells that do not form reproductive cells, through lateral gene transfer. David Riley and Karsten Sieber published the result of a study indicating that more than 7,000 events of lateral gene transfer have occurred from bacteria to humans. Interestingly, though, almost all of these lateral gene transfers were found to occur in cancerous cells [7]. A similar mechanism has long been known to exist with viruses and insertion of viral DNA into human somatic cells has been directly implicated with cancer development.

Unlike bacteria, viruses were able to insert their genes into the genomes of human egg and sperm cells. Frank Ryan’s book Virolution extensively discusses this phenomenon [8]. The virus DNA can be passed down from the parents to offspring, allowing the integrated virus DNA to be replicated and reinserted into the ancient human genome. After thousands of generations and hundreds of thousands of years, 8% of human genome is now made of viral genes. This is quite interesting given that the original human genes that serve as the blueprint for our body’s overall development and functioning make up a meager 1.2% of the human genome! Do viruses hold a special place in the evolution of life on Earth? For example, did viruses have a role in the evolution of the most sophisticated organisms on this planet, us humans?

References

1. Fidler, A. E., van Oers, K., Drent, P. J., Kuhn, S., Mueller, J. C., and B. Kempenaers (2007). Drd4 gene polymorphisms are associated with personality variation in a passerine bird. Proceedings. Biological Sciences 274(1619): 1685–1691

2. Thomson, C.J., Hanna, C.W., Carlson, S.R., Rupert, J.L. (2013). The -521 C/T variant in the dopamine-4-receptor gene (DRD4) is associated with skiing and snowboarding behavior. Scandinavian Journal of Medicine & Science Sports 23(2): e108-e113

3. Kourt, A. (2017). The curiosity gene: On the origin of humankind by means of intrinsic motivation. US: CreateSpace Independent Publishing Platform

4. Byrne, R.W. (2013). Animal curiosity. Current Biology 23(1): R469-R470

5. Gräslund, B. (2005). Early humans and their world. UK: Routledge Publishing

6. Wesołowska, W. and T. Wesołowski (2014). L. eucochloridium sporocysts and snail host behaviour. Journal of Zoology 292: 151–155

7. Riley, D. R., Sieber, K. B., Robinson, K. M., White, J. R., Ganesan, A., Nourbakhsh, S., J.C. Dunning Hotopp (2013). Bacteria-human somatic cell lateral gene transfer is enriched in cancer samples. PLoS Computational Biology 9(6): e1003107

8. Ryan, F. (2009). Virolution. London, UK: Collins Publishers