LITMIR.BIZ

1 The Tree of Life (I)

      1.1 Introduction

      This chapter provides an overview of life and draws near some important questions: When did life on earth begin? What is life? How is it organized? When did multicellular organisms appear and why? How many species exist on Earth? Notions of biology related to the emergence and classification of life are discussed in connection with different strategies on organism formation. Some ultrastructural images (electron microscopy) are presented as examples for reference. The lower and upper physical dimensions of eukaryotic and prokaryotic organisms are explored in detail. The same exploration is made for viruses that interact within the kingdoms of life (Animalia, Plantae, Fungi, Protista, [Archaea and Bacteria] or Monera). Moreover, a discussion closely debates the reference system and the requirements for life; with special considerations for the “spark of metabolism.” Next, an introduction is made on some concrete topics, namely: The origins of eukaryotic cells, the endosymbiosis theory, the origins of organelles, the notion of reductive evolution, and the importance of horizontal gene transfer (HGT). Toward the end of the chapter, the main hypotheses regarding the origin of eukaryotic multicellularity are explored using the behavior observed in current species.

1.2 Emergence of Life

The Earth is believed to be ∼4.5 billion years old. Geological evidence shows that liquid water, continental crust, and a rudimentary atmosphere existed on Earth just 100 million years later (4.4 billion years ago) [1]. The planetary atmosphere consisted of water vapor, carbon dioxide, methane, and ammonium [2]. It is unknown exactly when or how life began on Earth [3]. It is considered that life began on the early Earth soon after conditions became favorable for a chain of consecutive, yet undetermined chemical reactions [4]. The field of prebiotic chemistry tries to explain how organic compounds formed in the absence of biology and how these simple molecules self-assembled to ignite life on Earth and possibly on other planets. The oldest fossils of single-celled organisms date around 3.5 billion years ago [5, 6]. Nonetheless, only organisms with a dense biological structure would have resisted the intense metamorphism experienced by crustal rocks for more than 3.5 billion years. In turn, a dense biological structure may indicate high organism complexity. Thus, the earliest known microfossils could actually indicate the presence of structurally complex unicellular organisms [7]. It stands to reason that those “first” organisms must have required a long time to develop their complexity. Evidence for life on Earth before 3.8 billion years ago has been proposed in the past [7]. Preserved carbon, potentially of biogenic nature, pushes the origin of life on Earth to 4.1 billion years [8]. This indicates that life may have occurred fairly quickly after the formation of the planet (4.5 − 4.1 = 0.4 or 4.5 − 3.8 = 0.7). That is, 400 million to 700 million years after the formation of the planet. Moreover, the observation has important implications for our beliefs about how fast life ignites on other planets with similar conditions. In the next important event, life brings chemical modifications to the planetary atmosphere. An oxygen-containing atmosphere and evidence of cyanobacteria and photosynthesis date around 2.4–2.2 billion years ago [9, 10]. Large colonial organisms with coordinated growth in oxygenated environments have been found as far as 2.1 billion years ago [11]. Life made a gradual step toward eukaryotic unicellular organisms ∼2 billion years ago. Eukaryotes divide into three main groups around 1.5 billion years ago, namely in the unicellular ancestors of modern plants, fungi, and animals [12, 13]. The appearance of multicellular eukaryotes is an unclear period. During evolution, gain or loss of multicellularity often occurred until a stable multicellular state was reached [14]. Knowledge of the complexity and size of current single-celled eukaryotic organisms calls into question many more complex fossils. It is difficult to investigate whether some macroscopic-sized

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