A Singular Work of Theater

Abstract

The history of biodiversity on the planet Earth could be compared to a work of theater. Biodiversity is a word which has been on the lips of many, often with a note of concern in the speaker's voice. Yet what is biodiversity? The word simply denotes the variety of biological organisms. When you walk through a forest, when you dive into the sea, when you roam through a wheat field, relax on the beach or trek through a desert, you can see this biodiversity all around you. Almost by definition, it is not identical everywhere: sometimes ubiquitous, sometimes more discrete, but always present on (or near) the surface of our planet and in the oceans. It has been this way for a long time. For billions of years, even. Thus, biodiversity has a history - a very long one. In order to correctly and accurately analyze this biodiversity, it is necessary to look at its historical aspect, in context. With over 270,000 recorded species, flowering plants are, indubitably, a major part of the biodiversity present in the world today - all the more so when one considers that practically all vegetable material that we use in human foodstuffs comes from flowering plants. How come they are so diverse? When did this situation arise? Without understanding their diversification over the eons through geological records, we have no hope of accurately grasping the extent, nature and significance of their diversity today.

Keywords

Biomineralization

Diversification

Evolutionary radiations

Fossils

Mammals

Mass extinction

Natural selection

Precambrian/Phanerozoic shift

1.1 A unique history

The history of biodiversity on the planet Earth could be compared to a work of theater. Biodiversity is a word which has been on the lips of many, often with a note of concern in the speaker's voice. Yet what is biodiversity? The word simply denotes the variety of biological organisms. When you walk through a forest, when you dive into the sea, when you roam through a wheat field, relax on the beach or trek through a desert, you can see this biodiversity all around you. Almost by definition, it is not identical everywhere: sometimes ubiquitous, sometimes more discrete, but always present on (or near) the surface of our planet and in the oceans. It has been this way for a long time. For an extremely long time. For hundreds of millions of years. For billions of years, even. Thus, biodiversity has a history - a very long one. In order to correctly and accurately analyze this biodiversity, it is necessary to look at its historical aspect, in context. With over 270,000 recorded species, flowering plants are, indubitably, a major part of the biodiversity present in the world today - all the more so when one considers that practically all vegetable material that we use in human foodstuffs comes from flowering plants. How come they are so diverse? When did this situation arise? Without understanding their diversification over the eons through geological records, we have no hope of accurately grasping the extent, nature and significance of their diversity today.

The idea I am attempting to put across in this book is a simple one: the story of biodiversity is, above all, a story of diversifications! Certainly, it is a story shot through with instances of extinction - sometimes by rather violent means. Yet the most striking feature of biodiversity is its incredible capacity for diversification. In scientific terminology, such diversification, when it is particularly significant, is called "evolutionary radiation". This term will be used abundantly in this book. It is worth remembering: it denotes events of diversification of life on Earth. The study of evolutionary radiations is at the heart of this book.

Let us go back to the point made at the start of this chapter: the history of biodiversity on the planet Earth could, to a certain extent, be viewed as a work of theater. It has a beginning, a succession of "acts", various actors - some at the head of the bill, and others with a more secondary role. Just like a play, the events take place against a changing backdrop. The position of the continents, the average sea temperature and the prevailing ocean currents are all elements of this "set" (among many others), which change over time. However, this resemblance with a work of theater is only superficial. Unlike a play, the story of biodiversity is not written in advance by a responsible individual. Indeed, it is not written in advance at all! It is, by its very nature, contingent. The events which occur on the evolutionary "stage" are primarily attributable to chance. A geological phenomenon, such as the opening of an oceanic rift, can give rise to changes in the environment which will play a role in the process of natural selection. Certain species encountering these new conditions will be able to adapt, whilst others will be driven to extinction. Those same species who do manage to adapt may then become extinct if the environmental conditions change again. The splitting of a geographic area into two - whatever the mechanism that causes it - may divide the population group of one species, and lead to the emergence of two new species. Random chance is a majorly important player in this work of theater. Thus, it renders the story entirely unique. Travel back in time, to the same exact conditions of the beginning of life on Earth, around 3.5 billion years ago, with the same actors and the same setting as before. Dim the house lights and raise the curtain, and let the action play out again. In all probability, you will see an entirely different story. This is the idea championed by Stephen Jay Gould (1941-2002), a renowned American paleontologist [GOU 89]. Although there are clearly demonstrated mechanisms which help to shape biological evolution - of which natural selection is one example - it is nonetheless true that evolution is, by nature, contingent.

Today, numerous academics are engaged in imagining the evolution of biodiversity in days to come. These projections are made over relatively short periods of time, and use the same actors (the species which are around today) and the same elements of set (the present-day environment), which they alter in accordance with various scenarios. In 2009, for example, Cheung and his colleagues [CHE 09] calculated the effects of the climate changes likely to occur by 2050 on the distribution of over 1,000 species of marine animals (primarily fish). One of the lessons from this study is the prediction of numerous local extinctions of species in certain geographical zones - mainly the subpolar and tropical regions. Studies such as this one are increasingly prevalent in academia today. They enable us to better understand the effects that the coming environmental changes are likely to have on biodiversity. However, it must be noted that these studies are very greatly focused on the near future: projections over a few dozen years at most. Making projections beyond that remains a risky business - very risky, even. Unpredictable events (those which are, by nature, linked to random chance) may considerably impact the performance of the projection models.

However, the future state of Earth's biodiversity is not the only issue worthy of interest. The biodiversity of the past is just as fascinating an object of study. The discoveries made by paleontologists looking at life in the past are far beyond what anyone could imagine. This exploration of the past helps answer the primary question of paleontology:

- what has been the history of life on our planet?

This central question invites other questions, some of which fit in entirely with the concerns of society today:

- what were the different actors that have played on that stage throughout the ages?

- how did some of those actors come to disappear?

- how does this past biodiversity illuminate what we know about biodiversity today?

- could the events of the past shed some light to help us better predict the future evolution of biodiversity on our planet?

Today, we can trace the outlines of the history of biodiversity thanks to fossils which bear witness to this past life. Fossils are the remains of ancient organisms or the traces of their activities: remnants in the form of bones, shells, teeth or traces of movement or predation, for example. Paleontologists discover and study fossils - not only to obtain a catalog of the most bizarre, most enormous or most ferocious forms of life, although this is an undeniably enjoyable activity! They study fossils in order to answer one of the greatest questions in modern science (posed a few lines earlier) what has been the history of life on our planet? Using their research, paleontologists reconstitute and order the different acts in that story, and the actors that have played out this singular piece of theater. It is by collecting fossils in the field that we are able to find out about the different actors. Yet this act of collection, however abundant, is not sufficient to precisely reconstruct the story. Paleontologists have only recently revealed their synthetic reconstructions of the history of life on Earth to the eyes of the public. By compiling the successive discoveries of fossils...