How did life start?

The Earth is thought to have formed about 4.5 billion years ago and was initially devoid of life. There appears to be evidence of bacteria-like organisms in rocks laid down approximately 3.5 billion years ago. If these microfossils are actually very early prokaryotes (see below) then it appears that life must have started within the first billion years of the Earth’s existence. The move from a lifeless planet to one now teeming with life is thought to have occurred through a series of major phases (Fig 11-1). Initially, conditions on the primordial Earth were very harsh but were ideal for spontaneous reactions between hydrogen, carbon and nitrogen to occur, leading to the production of ammonia and methane and, later, more complex organic molecules. Eventually the conditions moderated to allow large volumes of liquid water to exist, giving a medium in which reactions between these more complex organic molecules could occur spontaneously. The initial absence of oxygen in the primitive atmosphere was advantageous in that it allowed the newly-formed molecules to be more stable since these reductive conditions permitted large quantities of these molecules to build up, and because oxidation is often deleterious to biological molecules. This led to the development of a non-living ‘primordial soup’ rich in organic molecules.

FIGURE 11-1 A scheme showing important phases in the evolution of life. The Earth formed about 4.5 billion years ago, producing a lifeless environment. Over the next few hundred million years simple molecules were converted into more complex organic molecules which began to accumulate. The next step was probably the formation of simple cell-like structures (protocells) which later gave rise to the first prokaryotes. About 3 billion years ago photosynthetic bacteria started to produce oxygen which accumulated in the atmosphere, and about 2.5 billion years ago the first eukaryotes evolved out of the more complex prokaryotes.

The next key step was to condense these molecules to give macromolecules. It is believed that over hundreds of millions of years the amino acids and other organic molecules originally produced in the prebiotic stage of the Earth’s existence condensed to give simple proteins, phospholipids and nucleic acids. These molecules became sequestered in membrane-bounded vesicles to generate protocells. Gradually the chemical reactions occurring in the protocells became sufficiently organised for their transition to what can be considered as the first ‘living cells’. Two important facets for this transition were gaining the ability to: (1) capture and harness energy from the environment so that they could carry out synthetic reactions (see Ch 17); and (2) store, replicate and utilise information (see Chs 20 and 21) to make proteins, which became the cellular catalysts to help reactions occur more easily. The appearance of living cells led to an alteration in the driving force behind the changes occurring. Initially, change was driven purely by chemical reactions occurring spontaneously whereas the development of living cells enabled them to pass on a biological blueprint to offspring (see Ch 22), beginning the process of biological evolution—the change in the inherited traits of organisms through successive generations.

Biological evolution, in turn, led to the appearance of all the major features of cellular life. The driving force of evolution is natural selection of advantageous traits. In other words, when a particular organism acquires a novel characteristic that offers it an advantage over those that lack it, that organism reproduces more efficiently. Eventually that advantageous trait becomes common in a population of organisms. Of particular importance in evolutionary terms was the development of a nuclear region to store information as DNA, and systems to copy the DNA, and convert the information it carried to RNA for use in protein synthesis (see Ch 20).

Another important step was the development of ribosomes and the associated enzymes needed to make proteins. To do this, the cells also needed an oxidative system to supply chemical energy for use in protein synthesis and other synthetic reactions occurring in the cells. Later cell division developed to allow an increase in cell number in a manner that evenly distributed the information stored in the DNA to all the daughter cells. The membrane bounding the cell also gradually gained functionality so that it could control the movement of molecules into, and out of, the cell. All of these developments probably occurred in the first billion years after the Earth’s formation. These earliest cells were probably very simple prokaryotes (see below).

About a billion years after the appearance of the earliest prokaryotes there is evidence that the first eukaryotic cells appeared. Modern eukaryotes can be differentiated from prokaryotes because of: (1) the separation of DNA from the rest of the cell by a nuclear membrane; (2) the presence of membrane-bound compartments with specific functions, for example, mitochondria, chloroplasts, endoplasmic reticulum; and (3) specialised proteins that move cellular components or the cells themselves.

It is believed that eukaryotes developed because of associations between early prokaryotes. For example some organelles such as mitochondria and chloroplasts may have originated from endosymbiotic relationships between two prokaryotic cells. The endosymbiotic theory hypothesises that mitochondria may have developed when photosynthetic and non-photosynthetic prokaryotes coexisted in an oxygen-rich atmosphere. These non-photosynthetic prokaryotes fed themselves by ingesting organic material, which probably included other cells, from their immediate environment (Fig 11-2). This made them the earliest predators. These predatory prokaryotes probably included both anaerobes (organisms that do not require oxygen for growth, can react negatively or may even die in its presence) that could not utilise oxygen in energy production and were therefore unable to fully capture their inherent energy, and aerobes (organisms that can survive and grow in an oxygenated environment). It is believed that among the cells that were ingested were some aerobic cells, which instead of being digested, persisted in the predatory cells. These persistent aerobes became endosymbionts, living symbiotically within the other cell.

FIGURE 11-2 The endosymbiont theory of how eukaryotes got some of their organelles. One theory of how some of the eukaryotic organelles evolved is based on the idea that early prokaryotes may have formed symbiotic relationships. Some early prokaryotes took up a predatory lifestyle, getting their energy by absorbing organic molecules and other prokaryotes from the environment. These were probably anaerobic organisms which could not use oxygen in their metabolism. Some of the cells they ingested were aerobes which could use oxygen. It has been suggested that some of these cells may have persisted in the predatory cells instead of being digested and that they later evolved into mitochondria.

In the transition to a recognisable eukaryotic cell, a prokaryotic cell also needed to acquire other membrane-derived structures such as the endoplasmic reticulum and the Golgi complex. It is believed that pronounced infolding, known as invagination, of the bounding membrane may be responsible for the evolution of these structures. It is unlikely that they originated from endosymbionts. Based on fossil evidence, these changes to convert prokaryotes into eukaryotes took about 1.3 billion years. It is possible that eukaryotic cells evolved earlier but the evidence for this has yet to be found. The first eukaryotic cells had now appeared, and would become the ancestors of all modern eukaryotes.

The main system of classification used by scientists today groups all living organisms into three domains (see below, Table 11-1 and Fig 11-6). One of these domains, the Eukarya, contains all the eukaryotes. The second domain, the Bacteria, contains both photosynthetic and non-photosynthetic prokaryotic bacteria. The final domain, the Archaea, contains bacteria-like prokaryotic organisms that inhabit extreme environments such as hot springs and thermal vents in the deep ocean. All three domains share common fundamental characteristics; they use the same genetic code, and DNA and RNA molecules carry out the same basic functions. This makes it likely that they all evolved from a common ancestral cell line. How Archaea fit into the evolution of both bacteria and eukaryotes has yet to be elucidated since they share characteristics of both groups of organisms.

FIGURE 11-6 Different ways in which organisms can be categorised using taxonomy. There are several ways in which living organisms can be classified in groups. In this chapter the three-domain system, in which all life can be placed into three domains, is used. In the most complex classification system the Eukarya can be split into four kingdoms, which together with the Bacteria and Archaea, give a six-kingdom model. This can be converted to a five-kingdom system if the Bacteria and Archaea are combined into the Monera (Prokaryotes).

The evolution of multicellular life

The first eukaryotes were unicellular organisms but later gave rise to multicellular versions. Molecular analysis of modern eukaryotes suggests that the first multicellular eukaryotes appeared about 900–1000 million years ago, and there is evidence in the fossil record of such organisms around 600–800 million years ago. It is thought that multicellular eukaryotes initially arose through cells of the same type congregating into a colony (Fig 11-3). Later the cells gained the ability to act in a coordinated manner such that these colonies were better able to adapt to environmental changes. Subsequently, cells within the colonies differentiated into specialised cell types with diverse but distinct functions. This gave these colonies a wider range of capabilities and adaptability. Over time the division of function among cells led to the evolution of the tissues and organs of complex eukaryotes.

FIGURE 11-3 A scheme showing important steps in the development of multicellular life. The earliest life is believed to have been unicellular. Later it is likely that cells of the same type were able to aggregate into colonies. These colonies initially contained cells that were exactly the same. Finally cells in the colonies developed separate functions and later developed into the tissues and organs of more complex organisms.

Living organisms: classification and naming

Estimates of the number of different types of organisms on our planet range from 5 million to over 10 million, but only 1.7 million have been definitively characterised. The best known, and most studied, are the birds and mammals, which account for less than 0.1% of the total. Insects (∼65%) and fungi (∼8%) have been partially described whereas other groups such as soil nematodes, protozoa and bacteria are mainly unnamed and undescribed.

The relationship between a series of organisms can be predicted by a phylogenetic tree. The branched structure of these trees is made by comparisons of characteristics between the organisms, and can be done in several ways:

FIGURE 11-4 A phylogenetic tree of monkeys and apes derived from molecular analysis. Using comparisons of the sequence of a protein or its gene or the sequence of ribosomal RNA it is possible to gain an understanding of the evolutionary relationships between species.

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