Saturday, June 6, 2009

Phagocytotic Predators from the Proterozoic?

ResearchBlogging.orgBy way of simulation, a recent addition to PLoS One’s Paleontology Collection examines the rise of predation as a feeding mechanism in unicellular organisms. Taking what I would describe as a theoretical perspective, the authors evaluate the question of predatory origins in the light of recognized ecological and life history traits exhibited by modern eukaryotes. Central to their argument is adherence to the following fundamental biologic and physical principles: Conservation of energy and matter, Surface to volume ratio, Power law of metabolism, Inheritance, Predation advantage, Size asymmetry, and Positional information.

Though I would agree with the paper’s authors in that a biologist might find it “…suspicious that there would be any period of time, however remote, that normal biological and ecological principles did not apply,” I would also emphasize that ecological and biological interactions tend to be constructed from compounding processes that develop with specific respect to time. And, when we examine deep geologic time it’s important to bear in mind that although the rules of the game remain constant, the players may have been substantially different – different in quantity, diversity, intra/inter-population density, etc…

These factors are absolutely crucial in the advent of any competitive strategies, rather they are for energy acquisition as in predator-prey relationships, or locomotion/movement tactics as methods for searching and dispersal. The question isn’t so much “did predation arise through the interplay of natural forces” – we’re fairly confident of that – instead the question is, “what is the duration and timeline associated with these processes?”

At this risk of sounding overly critical, I think that there may be a disconnect between the paper’s introductory assumption that predation arose in less than about 1–3 billion years, and the bulk of the paper’s data which argues that predation is the result of natural processes. Somewhere during the read, it seems to me, the perspective of time is lost...

It’s not unreasonable to suggest that at some point in deep geologic time there existed an "original" population of protozoans (or other unicellular critters), and that this population consisted of numerous, multiplying individuals whom expended energy. Due to the physical constraints of chemistry and the physiology of biological metabolism, generating energy requires the continual acquisition and consumption of resources from the surrounding environment. Unfortunately, the surrounding environment doesn’t possess unlimited resources, thus populations do not experience unbounded growth and reproductive autonomy - there are restrictions to population size.

As the population of protozoans reproduced and expanded, shortfalls in available resources would cause the population to undergo a “leveling out effect” (or rather, enter a state of oscillation) in which the number of individuals present would hover near the environment’s carrying capacity. At this point, the population either randomly fluctuates near this carrying capacity, while awaiting its inevitable demise at the hand of environmental change; or, the individuals in the system attempt to locate new, or alternative, resources through the mechanisms of natural selection.

In order for the population of protozoans to track down additional resources, they must either expand their range through greater exploration and dispersal; or, change in such a way as to enable the consumption of other, more prevalent resources – through predation or otherwise. Either of these options, range expansion or the processing of alternative foodstuffs, require morphological adaptation, the most basic of which is simply growing bigger.

Individual protozoa are far too small to be effective range expanders; therefore an increase in an individuals overall size (as well as propulsion) would be necessary to maximize travel efficiency. Likewise, in order to process new foodstuffs an organism must develop both a new means of acquisition (or gathering) and a new means of metabolizing the “foodstuffs” once they are found. To a certain degree, this implies a change in morphology and physiology – new parts and the metabolism to support those new parts are needed.

(NOTE: Perhaps a population can expand a range over multiple generations, but on my assessment adaptations “for the good of” a species or population don’t have as much explanatory torque; that’s another topic though - my point here is that individual protozoa would have some difficulty exploring the seas in search of palatable sustenance.)

As an organism grows bigger - increases its volume - it must do so while obeying certain physical and biological principles. For instance, any increase in total volume must be accompanied by a proportionately greater increase in available surface area. The surface of an organism is where energy absorption and environmental interaction occurs, consequently surface area must increase to permit inclusion of ample energy, to facilitate ecological interaction and to sustain the internal components. (For example, if an organism were spherical, volume would increase as the cube of the radius, whereas the surface area would only increase as the square of the radius; so, in order to maximize energy absorption the organism must either develop in an elongate fashion, or develop additional specialized structures such as appendages, trichomes, folded skin, etc.).

Such a drive to acquire resources may have enabled protozoa (or other unicellulars) to grow or adapt in a way as to permit phagocytotic predatation, but to the best of knowledge, that isn’t in doubt - what is uncertain is the timing of such events. How long did it take for the “original population” to reach a point of ecological pressure sufficient to drive a transition towards predation – how much TIME ???

To end on a positive note however; throughout the paper the authors emphasized the need for scientists studying the biologic systems of deep geologic time to account for established biologic and ecologic principles during their research – this point is clearly supported by the current paper and I think that it will be well received by most!

de Nooijer, S., Holland, B., & Penny, D. (2009). The Emergence of Predators in Early Life: There was No Garden of Eden PLoS ONE, 4 (6) DOI: 10.1371/journal.pone.0005507

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