Origin of Cannibalism?

1100 Words, 5 minute read

Scientists at Rensselaer Polytechnic Institute (RPI) recently discovered a microscopic organism, Euplotes Gigatrox(EG), that can change size, shape, and behavior to hunt and consume their genetically identical relatives. In other words…cannibals!

Under specific conditions, a small number of cells in clonal EG colonies ‘spontaneously’ develop into super-giants more than twice the length of normal cells, with a broader body shape and a larger mouth. These altered cells become predators, capturing the now smaller clonal relatives and swallowing them whole, at a rate of roughly one prey every ten minutes. 

Beyond the obvious horror involved in this practice, this phenomenon demonstrates that single-celled organisms are capable of complex behavior previously studied only in multicellular animals.

EG has discovered a feedback loop that allows it to continuously optimize a colony’s population density in response to ever changing environmental conditions. Imagine no more famines and no more one child policies! (Of course, these gains come at an horrific price!)

EG’s discovery raises more questions than it answers. How does the colony control the behavior of its individual members so that the aggregate result is optimized for the survival and procreation of the colony? How is it decided which organisms will transform (predators) and which will remain in their natural state (prey)?

How does the colony know when the concentration of super-giants is optimal and how is that determination communicated among the individual organisms so that no additional cells convert to super-giant status? Likewise, how does the colony know when the crisis has passed and the concentration of super-giants must be reduced, ultimately to zero. How are individual super-giants instructed to revert to their normal state?

The RPI team sequenced single-cell transcriptomes from (1) normal cells, (2) super-giants, and (3) cells that had recently reverted back to ‘home base’ from a super-giant state. The results showed that super-giants are a distinct developmental stage, with widespread differences in gene expression including cell cycle regulation, protein production, and membrane organization.

I am reminded of the Caterpillar whose genetic material completely reorganizes during its metamorphosis into a Butterfly.

Cells that revert from the super-giant state also carry a distinct molecular signature that appears to temporarily suppress the pathways driving transformation. Populations started from recently reverted cells produced new super-giants more slowly and at lower overall rate than populations started from normal cells, regardless of external conditions. 

This makes perfect sense! If the initial population adjustment overshoots the mark, it makes sense that the colony would be more restrained in the future. Somehow, this colony of single celled organisms ‘learns’ from experience and adjusts its future behavior accordingly, a feat not always duplicated in humans. 

Super-giant formation tends to occur following periods of rapid population growth, especially when alternative small prey are less than abundant. Formation continues only while alternative prey remains scarce. Importantly, super-giants never exceed about 5% of the population.

5% appears to be a universal tipping point for all EG populations. A concentration of super-giants in excess of 5% almost always results in depletion of the population below safe levels. Unlike most gluttons (present company included, non-cannibalistically of course), EG knows its limits.

This itself is a puzzle. How can a species learn not to ‘extinct itself’? After all, we’ve been told that ‘extinction is forever’: you don’t come back from an extinction event so how can you learn from it. Arguably at least, Homo Sapiens(HS) is struggling with a similar paradox right now. The existential question: Will we be as successful as our unicellular cousins?

There are two fascinating aspects to this discovery. First, it offers a clear model of population (vs. organism) centric evolution. Second, it bridges a long supposed gap between unicellular and multicellular organisms.

The evolution of life on Earth traces an interesting, and I think somewhat unexpected, timeline. Give or take a few hundred million years, but who’s counting, the sequence of events looks something like this:

                                    4.5 billion (years ago)                  Earth takes solid form

                                    4.0 billion                                              First self-replicating molecules (RNA)

                                    3.5 billion                                               First cell (DNA powered)

                                    2.0 billion                                               First eukaryote (nucleated cell); e.g. EG

                                    1.0 billion                                               First multicellular organism

                                    0.5 billion                                               First animals; e.g. sponges

                                    .05 billion                                               First primates; e.g. HS

This discovery (Euplotes Gigatrox) pushes back the evolutionary timeline for certain functional behaviors by 400%. It’s as if we just found out that George Washington was not born 400 years ago but 1600 years ago, before the Fall of Rome. Or that Bill Gates invented the Windows operating system during the War Between the States. Unsettling.

Yet somehow these unicellular organisms are able to carry out all the functions of both a cell and an entire organism within a single membrane. 

According to Ben Larson, a member of the RPI research team, “Most of what we know about development comes from animals.  We now have a system where we can study those same fundamental questions, as analogous developmental processes play out in a single-celled organism on a completely different branch of the tree of life’.” 

Fascinating…but so what? Well first, perhaps we can learn something from our unicellular ancestors: how to prioritize population survival over individual success. But second, this is another example of the incredible collapse of the Tree of Life that is taking place in our own time.

Just a few generations ago, HS was the star atop the Tree of Life. It was the purpose of the tree, its raison d’etre. It was the entirety of the tree compressed, i.e. the business end of an ontological telescope. The rest of tree, from baboon to bacterium, had no intrinsic importance and only functional value.

In my lifetime alone, the Tree of Life has transformed from a formidable redwood to a cute bonsai. The only question now is, “When will this process end?” The discovery of Euplotes Gigatrox suggests that we still have some way to go. What a time to be alive! What a time to be alive?

Photo Source: Ben Larson and Samuel Lord: Euplotes Gigatrox cell, imaged by scanning electron microscopy (SEM), published on the cover of the Proceedings of the National Academy of Sciences (PNAS).