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Earth is approximately 4.5 billion years old. Remarkably, there is convincing fossil evidence that organisms
morphologically (and very probably biochemically) resembling certain modern bacteria were in existence 3.5 billion
years ago. With the use of the results of directed studies and accidental discoveries, it is now possible to construct a
hypothetical yet plausible evolutionary path from the prebiotic world to the present. A number of uncertainties remain,
particularly with regard to the earliest events. Nonetheless, a consideration of the steps along this path and the
biochemical problems that had to be solved provides a useful perspective from which to regard the processes found in
modern organisms. These evolutionary connections make many aspects of biochemistry easier to understand.
We can think of the path leading to modern living species as consisting of stages, although it is important to keep in
mind that these stages were almost certainly not as distinct as presented here. The first stage was the initial generation of
some of the key molecules of life nucleic acids, proteins, carbohydrates, and lipids by nonbiological processes. The
second stage was fundamental the transition from prebiotic chemistry to replicating systems. With the passage of time,
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these systems became increasingly sophisticated, enabling the formation of living cells. In the third stage, mechanisms
evolved for interconverting energy from chemical sources and sunlight into forms that can be utilized to drive
biochemical reactions. Intertwined with these energy-conversion processes are pathways for synthesizing the
components of nucleic acids, proteins, and other key substances from simpler molecules. With the development of
energy-conversion processes and biosynthetic pathways, a wide variety of unicellular organisms evolved. The fourth
stage was the evolution of mechanisms that allowed cells to adjust their biochemistry to different, and often changing,
environments. Organisms with these capabilities could form colonies comprising groups of interacting cells, and some
eventually evolved into complex multicellular organisms.
This chapter introduces key challenges posed in the evolution of life, whose solutions are elaborated in later chapters.
Exploring a possible evolutionary origin for these