In this edition I have had two intentions especially in mind: to try to bring to life for the reader the Achaemenid empire, and to offer a good deal of help with the grammatical aspects of the text. The first intention responds to a growing interest in Greece's relationships with the Ancient Near East, and will I hope prevent the commentary and its readers from taking too Hellenocentric a view of Herodotus' account. That Herodotus makes a strong distinction between ‘Greeks’ and ‘Persians’ is an idea that is slowly being revised, as the complexity of his presentation is more and more explored. The second intention responds to my experience at the JACT Greek Summer School, held annually now at Bryanston School, in Dorset. I am very grateful to my various students there not only for making it clearer to me what is required in a modern commentary on a classical text, but also for permitting me to try out on them earlier drafts of the commentary.

Although a new text of Herodotus, based on fresh study of the MSS and a consideration of the linguistic problems involved in constituting such a text, is much to be desired, the text offered here is not the result of a new inspection of the MSS, but aims to be an accessible and readable text.

Introduction

While the notion of worlds beyond our Earth is ancient, the specific idea of planets orbiting distant stars is relatively new. Over two millennia ago Epicurus stated “there are infinite worlds both like and unlike this world of ours,” but he was not speaking of Earth-like planets orbiting Sun-like stars. Indeed, planets orbiting stars would have been a meaningless issue to the Greeks, as the Sun was not recognized as a star, nor the Earth as a planet (Chapter 1).

One of the earliest and most eloquent spokespersons for what is now called astrobiology, and among the first to grasp the implications of the Sun being a star and the Earth a planet, was the mystical Roman Catholic monk Giordano Bruno. In On the Infinite Universe and Worlds (1584) he wrote:

Bruno then concludes with the revolutionary slogan:

Bruno's reward for this prescience and for other heresies was condemnation by the Church, followed by immolation in a public square in Rome in 1600.

Introduction

Prokaryotic microorganisms were the only form of life for at least 80 percent of our evolutionary history (Schopf and Packer, 1987). Multicellular organisms including plants, animals and fungi evolved a mere 0.5–1.0 Ga from single-celled eukaryotic ancestors. Geologists and paleontologists debate the age of life on this planet and when the major microbial lineages first diverged (see Chapter 12 for details). Cyanobacterium-like fossils suggest that life emerged at least 3.45 Ga (Schopf et al., 2002; Schopf and Packer, 1987), but the biogenic origins of these structures are contested (Brasier et al., 2002; Section 12.2.1). The chemical record documents prokaryotic metabolisms that may have existed 3.47–3.85 Ga (Mojzsis et al., 1996) and eukaryotic biosignatures that may be as old as 2.7 Gyr (Brocks et al., 1999). Yet, these are still imprecise interpretations (some might be more recent microbial contamination) and do not set absolute limits on the possible origins of life on Earth. Early periods of heavy bombardment between 4.1 and 3.8 Ga might constrain when life first appeared on Earth, although microorganisms living off chemical energy at kilometer depths could have survived even the largest impact events.

By the standards of multicellular plants and animals, single-cell organisms look relatively simple (Patterson and Sogin, 1993), yet they transformed the atmosphere, the waters, the surface, and the subsurface of the Earth.

Introduction

Picture a future triumph in robotic space exploration: in a complex mission to Mars, a sample has been collected from the martian subsurface near a newly discovered hydrothermally active site at 30°N latitude. Ten years in the detailed planning and execution, the mission's Earth return capsule with its sample canister has landed in the Utah desert. The sample is now under extensive analysis and testing in an ultra-clean containment facility – and initial observations have shown positive indications that it contains life. Only after later testing is completed, checked, rechecked, and repeated is it shown unequivocally that the lifeform contained within the sample is a soil bacterium common to the dirt of an old Soviet launch facility in Baikonur, Kazakhstan, and which has apparently been alive on Mars since a spacecraft crash-landed there in 1972 …

Or picture, as did novelist Michael Crichton (1969) in the very year of the first lunar sample-return mission (Apollo 11), a spacecraft returning to Earth containing a dangerous extraterrestrial organism – The Andromeda Strain – not related in any way to Earth-life and operating by rules scarcely understood even after hundreds of humans have met their grisly demise …

Once you have those events in mind, you are developing a feel for what planetary protection might be, and what it is meant to prevent.

Introduction

How does life begin? Can life arise elsewhere than the Earth? These questions are among the most fundamental and challenging in all of biology. Charles Darwin once wrote to a friend, “It is mere rubbish, thinking at present of the origin of life; one might as well think of the origin of matter.” (Letter to J. D. Hooker, March 29, 1863.) Darwin made this comment when the knowledge required to think about the origin of life and matter simply did not exist. Now, 150 years later, we understand much more. We know that new elements are constantly being synthesized by nuclear fusion of hydrogen and helium in the interiors of stars, then expelled into interstellar space when stars reach the ends of their lives. This matter is the source of new stars and planetary systems, and it is literally true that planets like the Earth and the biogenic elements that give rise to life are composed of “star dust” (Chapter 3). We also know that liquid water once existed on Mars, and perhaps still does beneath the martian surface, suggesting that microbial life may exist elsewhere than on the Earth. Probably most important is that we understand living cells in unprecedented detail, even to the point of knowing the entire sequence of three billion nucleotide bases in the human genome, and we have begun to manipulate the genetic blueprint of life.

Introduction

Habitable planets are those bodies that provide environments, materials and processes that are advantageous for the formation and long-term evolution of life. Understanding the processes that lead to the formation of such planets is a central issue in astrobiology. We are of course handicapped in this quest since Earth is the only example of a planet with proven habitability – the only one known to have provided thermal, chemical, and other physical conditions that allowed life to form and survive for ~3.5 Gyr.

This chapter emphasizes the formation of Earth-like planets, those with environments capable of supporting complex life comparable to Earth's plants and animals. The focus on life comparable to Earth's multicellular organisms is partly due to the practical consideration that we better understand the environmental constraints of such life. Despite this restricted focus, note that most astrobiologists consider that the dominant form of life in the Universe, as it has been on Earth over most of its history, is probably far simpler and more rugged, analogous to Earth's bacteria and archaea (Section 3.2).

This interpretation of habitability is highly Earth-centric and assumes that life elsewhere is similar to terrestrial life and requires environments similar to those of terrestrial organisms. The actual cosmic limits of life are of course unknown, but the Earth-centric view is a reasonable, albeit conservative, place to start. Until there are detailed data on other inhabited planets, discussions of extraterrestrial life would be prudently biased by what is known from our Earth experience.

Why is this question important?

In observing our vast Universe thus far, we have encountered life only on or near the surface of our home planet. Yet life in its properties and behavior is so different from the barren realms that we have surveyed elsewhere, that we cannot help but wonder how it first took root here, and whether things that we would consider alive exist elsewhere. The fossil record on Earth appears to extend to 3.5 Gyr (Schopf et al., 2002) and isotopic evidence suggests the presence of life several hundred million years earlier than that. Recently, however, this evidence has come into question (Brasier et al., 2002), so caution should be used in relying on these conclusions (Section 12.2.1). No hard evidence exists at all, however, concerning the mechanism by which life first began here.

Every human culture has felt the need to address this question, considering its importance in defining our place in the cosmos. In the absence of firm evidence, the door has been left open to a variety of answers from science, mythology, and religion, each defining our place in the Universe in different ways. I will follow a scheme put forward by the scientist and philosopher Paul Davies (1995: 21) and separate the competing points of view into three groups, called Biblical–Creationist, Improbable Event, and Cosmic Evolution.