Life on a roller-coaster

Life is not easy. It may not always be okay. The history of Life also had ups and downs like a roller-coaster, some moments with a great turn up of diversity and some moments with a breathtaking fall. Literally. These moments of abrupt decline in diversity are called mass extinctions when most species disappear.

 

Not all species survive the journey.

There is usually a normal rate of extinctions in Nature. This rate is called background extinction. Some species are extinguished by natural factors. However, mass extinctions are events in which at least half of the lineages are extinguished in a short period of time. Recalling that “short” on the geological scale can mean up to 15 million years. These events mark an important change in the formation of the biota. There were five big events of this size and they were called “Big Five”. The end of the Ordovician (440 m.y.a) and the end of the Devonian (375 m.y.a) had extinctions caused by glaciations; the extinctions of the end of Permian (250 m.y.a) and the end of the Triassic (200 m.a.a.) were caused by great volcanic activity; the famous extinction of the late Cretaceous (65 m.y.a.) was caused by the impacts of meteorites.

 

The geological eras. Pterosaurs not included.

The image of a destructive meteor is a very common in the popular imaginary, although it was officially proposed only in 1980. Despite this extinction that decimated the dinosaurs to be more striking, the mass extinction of the Permian was the worst of all. Between 80-90% of all species that existed died, ecosystems such as forests and corals were decimated and the Life as we know it almost got eliminated. Important lineages like sea scorpions and trilobites disappeared. This was the most dangerous time to go on Earth, but we survived.

 

R.I.P. – Rest in Permian

Illustrations of the past eras point too dramatic titles such as “age of the fishes”, “age of the dinosaurs”, “age of men” as if it were a succession of which team is winning. In fact, we are still in the “age of bacteria” that remains surviving all extinctions. There were still bacteria in the Ordovician, there were fish in the Jurassic, and there were birds in the Eocene. There are no evidences about obligatory progress and gain of complexity, it is just survival. In 5/6 of the history, Life was unicellular and interpreting large animals as powerful survivors is a bias of human observation. Diversity today is a consequence of several lineages that have survived in their own way, changing some anatomic details. There were several Noah’s arks going through mass extinctions. The ark of the vertebrates still survives since the Cambrian, but some of us ended up being left behind.

 

When bacteria ruled the Earth

Mass extinctions also bring an important lesson in humility. It does not matter much your skills, you need to be lucky. Chance plays a large role in the survival of species. A hairy lineage would do very well if the climate changed in a great glaciation, but would be bad if the climate changed to a desert. And it would certainly not matter if he lived in the slopes of an erupting volcano. It’s not always blind luck: large, rare, ultra-specialist species restricted to one local becomes an easier target. But there are no precise answers. Chance also causes the winners. Mammals coexisted with dinosaurs for 100 million years until a meteor exterminated the best competitors. And the dinosaurs themselves were already decreasing in the meteor epoch. In addition, a lineage of dinosaurs also survived the meteor, diversified and took advantage of the extinction of others: the birds. Birds are dinosaurs that survived, not for its skills, but for a great portion of luck.

 

This is not how good luck is achieved

Life is a branched pattern of ephemeral components in which lineages matter more than the represents. This is a story punctuated by tragedies, but always (for now) recovered the roles, even if it is with new actors. We are living in a time of great loss of diversity. In the Anthropocene, the nowadays age, there are destruction of ecosystems, global warming, pollution, and unbridled hunting. In relation to birds, we have lost 1.3% of all known species in the last 500 years. This rate is 26 times bigger than a normal background extinction rate. Apocalypse Now. Whether it is or not a sixth mass extinction (now, by our fault) it does not matter. We must confront the problem of loss of a large number of species in a much faster period. With the knowledge of the other extinctions we can guard us against what can happen. Our species is here for a time crumb and we can be extinguished much more easily than many other ex-winners. Trilobites, dinosaurs, Ammonites, they all reigned much longer than humans. If we do not put ourselves in our place, soon we will be in another.

 

“Those who cannot remember the past, are condemned to repeat it”

 

 

References:

Benton, Michael J., and Richard J. Twitchett. “How to kill (almost) all life: the end-Permian extinction event.” Trends in Ecology & Evolution 18.7 (2003): 358-365.

Gould, Stephen Jay. “The evolution of life on the earth.” Scientific American 271.4 (1994): 84-91.

 

 

Adaptation is complex

Adaptation is a term that emerged before evolutionary thinking. Creationists presupposed that individuals are perfectly adapted to their functions in nature. In this view, cats are perfectly adapted in all details for hunting mice. Born to kill. This line of reasoning of course overestimates the efficiency of individuals. Watching a single episode of Tom and Jerry may change this preconception.

Actual living beings do not play their roles perfectly

After Darwin and his theories, structures in living beings  began to be perceived as having been previously shaped by natural selection. The form of a structure is polished by natural selection to adapt to a specific condition. This process is essential to evolution and is the guiding principle behind modern Biology. But historically it is difficult to determine what is the cause if you only see the product or a consequence. It is no surprise that “Who Moved My Cheese?” was such a best-seller.

This is not a spoiler

Natural selection became the common-place explanation for pretty much all structures seen in nature. Some scientists argue that natural selection by itself is responsible for all biodiversity. Employee of the month. However, other forces such as genetic drift and self-organization are able to promote evolution. It is difficult to allege that a structure has been molded exclusively by one of these forces.

A specific structure or a gain in complexity may have been originated with other forces. Small populations are more affected by random events, a case in which the force of genetic drift arises. It is easy to tell a history that fits to (adapts to?) a pattern, claiming something was “obviously natural selection shaped”, but other forces may have acted. What is the need for a word that indicates the result of a process if you can’t determinate the process?

The processes of the past are nebulous

Complexity is not necessarily positive. Bacteria are doing pretty well, thank you. Which is more complex: an ant or an elephant? A cat or a mouse? Novel structures may have been originated by other forces such as self-organization. In a scenario where a gene product controls some other gene’s expression, a random nucleotide deletion between them can eliminate a structure but promote a link making a…promoter. The advantage of this union is evident and complexity is merging, but with an origin  that is independent from the classic natural selection process.

Increased cephalization is claimed as evidence for adaptation. But is not always better

The adaptation concept is more complicated when considered in a genome-wide scale. Genes interact with each other in a web-like pattern. Though individual genes may be an adaptation to their roles on the web, changes of relations alter their historical origin. The utilization of previously existing pieces is frequent. The term bricolage is often used:  the creation of a new whole from a diverse range of preexisting pieces. Evolution is more of a Lego joke than an engineer’s plan. In this context of new uses for old parts, adaptation is an idea with restricted explanatory powers. Adaptation is a complex term that does not cover a complex world.

Are these pieces adapted to these functions?

Literature:

Lynch, M. 2007. The frailty of adaptive hypotheses for the origins of organismal complexity. Proc. Natl Acad. Sci. USA 104 (Suppl. 1), 8597-8604

Wilkins, A.S. 2007. Between “design” and “bricolage”: genetic networks, levels of selection, and adaptive evolution. Proc Natl Acad Sci U S A. 104 (Suppl): 8590 – 8596.