Introduction
Exploring Biodiversity: Science Must Seize the Unknown 80%

We actually know far too little about biodiversity! We are idly living on the improved achievements of a period of intense exploration, which lasted from the 18th century through to the beginning of the 20th century, with the beginnings of "systematics".

The modern formalized description of the diversity of life was born at the beginning of this period, namely the famous Systema Naturae by Carl Linnaeus (1758). These first classifications were constructed on the basis of an implicit order in life, as perceived by precursor authors. This comparative perception and the linking of structures between different organisms are, indeed, at the heart of the origin of the theory of evolution (Le Guyader 2018; Montévil 2019).

While the progress of systematics waned at the beginning of the 20th century, general biology developed extraordinarily. It focused on the study of the laws of life through the study of a few organisms that imposed themselves as "models", from the vinegar fly to the white rat. Immense discoveries were made about heredity, the functioning of organisms and living cells, which today form the basis of our general knowledge (Mayr 1982). In comparison, the exploratory and still descriptive approach to the diversity of living organisms was gradually becoming obsolete; it suffered from enunciating particulars rather than the universals of general biology (Mahner and Bunge 1997; Grandcolas 2017).

Fortunately, the subsequent development of a comparative methodology and phylogenetic analysis revived this field and enabled it to make a strong contribution to modern evolutionary biology (Nelson 1970). Biology then rediscovered the diversity of organisms (Wilson 1988), making a new synthesis by considering the general laws of life and the diversity of their expression in living organisms (Grandcolas 2018).

The balance sheet of these past decades of exploration is both extraordinary, commensurate with biological diversity and these glorious periods of discovery ab nihilo, and disappointing, as we too often capitalize on a false feeling of déjà vu (Grandcolas 2017).

And yet, to give just one figure, we currently only know about two million living species, in other words, less than 20% of the 10 million species whose existence has been statistically inferred on numerous occasions (May 1988). Study after study on the many groups of organisms shows how much remains to be discovered, whether small or large, or near or far from us (e.g. Bouchet 2006; Vieites et al. 2009; De Vargas et al. 2015; Hawksworth and Lücking 2017; Nicolas et al. 2017). We still know very little about most of the so-called "species known to science". We only have a few lines in old publications which describe more than half of the macro-organisms (Troudet et al. 2017) and a few molecules, instead of whole phenotypes on uncultured microbes (Konstantinidis et al. 2017).

The issue now is to understand that it is essential to discover the unknown 80% of biodiversity for several well-defined scientific reasons, more than for the thirst for new knowledge or for a compulsive collection of new species.

First of all, the laws of life have rather varied degrees of generality; from heredity to the functioning of ecosystems, for example, there are several orders of magnitude of difference in this respect! Many laws or principles require the study of more particulars in order to reach generality, given the variation that is the intrinsic property of living things (Montévil et al. 2016). Clearly, we need to know about more organisms and the particular cases of their biology in order to be able to claim to generalize. The rules of representativeness, dominance or abundance, stated as truisms, are often misleading in living organisms. For example, it has recently been documented that rare and scarce species often fulfill disproportionately important functional roles within ecosystems (Mouillot et al. 2013).

In contrast to universals, particulars very frequently remind us of how many pathogenic, invasive species are discovered in this way, having already crossed half the planet, causing us great concern. This is as much the case for HIV (Barré-Sinoussi et al. 1983), an obscure retrovirus from an African primate, as for an invasive and unknown flatworm imported from South America, threatening the fauna of our soils and their faunal balance (Justine et al. 2020). Not a week goes by without a species new to science presenting a question to our societies. The COVID-19 pandemic is a dramatic demonstration of this: here again, a few poorly known bats and pangolins harbor unknown (and described for the occasion) coronaviruses whose genetic recombination is putting the human world at a standstill (Hassanin et al. 2020). This is also the case for viruses and viroids of plants that are still largely unknown, and vectors of devastation in some plantations (Maurel 2018).

The issue of bio-inspiration (Benuys 1997) is another opportunity to understand how much the diversity of living things contains wonders from which we can draw inspiration for more sustainable societies; so many particulars (structures, functions, etc.) in different species whose natural function can be transposed to functions of human interest. Practicing bio-inspiration beyond random discoveries of opportunity requires a broad and reasoned exploration of living things and the relationships between their structures and functions.

Particulars are also often geographical rather than purely taxonomic. Each state, government or municipality needs to be aware of local biodiversity in order to develop a reserve or environmental, agricultural or health policies (Pellens and Grandcolas 2016). These are all reasons to be aware of local fauna and flora with their innumerable numbers of endemic species (Caesar et al. 2017). It is worth remembering the order of magnitude of these numbers and that there are, for example, 40,000 species of insects in metropolitan France alone (Gargominy et al. 2014).

Even if we focus on a few species for reasons of immediate interest, it is essential to know their close relatives. Knowing the meaning, adaptive character and selection regime of the traits of organisms, whether they are genotypic or phenotypic models, requires an understanding of their history (Jenner 2006). Is it necessary, once again, to quote Dobzhansky (1973) - "nothing in biology makes sense except in the light of evolution" - to be convinced of this. Reconstructing the origin and evolution of the traits of an organism of interest requires knowing not only its close relatives, but also a very large part of the living world. How many fundamental traits has the human species inherited, the understanding of which is based on their structure and function at the Metazoan scale (more than a million species!)? This presupposes an adequate taxonomic sampling of life, which is not necessarily limited to known species, but which must be searched for out in the field in order to find unknown species whose lifestyles have sometimes been long surmised.

The entirety of this book is therefore dedicated to these approaches to exploring the diversity of life, each of them showing the crucial need we have for exploratory approaches. "Exploratory", which is easy to understand when reading this volume, does not refer to a kilometric description of specific characteristics, but to an organization of knowledge and hypothesis tests, based on a large sampling of living species - a large part of which is, strangely, still unknown to us, even though we come into contact with it every day. Without further delay, we must not suffer from or destroy biodiversity, but study it in order to integrate it sustainably into our societies.

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