the objects to hit Earth was so large — about the size of Mars — that it splashed material into Earth’s orbit that coalesced to form the Moon. The oldest rocks brought back from the Moon have ages measured to be 4.4 billion to 4.5 billion years. The oldest solid materials found on Earth are zircon crystals that formed 4.4 billion years ago. Rocks older than 3.5 billion years have been found on all the Earth’s continents.

Living things appeared in the first billion years of Earth’s history.

Evidence from the most ancient fossils reveals that life has existed on Earth for most of our planet’s history. Paleontologists working in Western Australia have discovered layered rocks known as stromatolites that appear to have resulted from the actions of bacteria at least 3.4 billion years ago, and fossils of cyanobacteria (also known as blue-green algae) have been determined to be nearly 3.5 billion years old. Other chemical evidence suggests that life may have originated much earlier, within a few hundred million years of when Earth’s surface finally cooled.

Modern stromatolites formed by single-celled organisms (inset) closely resemble the structures formed by some of Earth’s earliest living things.

Figuring out how life began is both an exciting and a challenging scientific problem. No fossil evidence of life forms older than 3.5 billion years has yet been found. Re-creating conditions that led to those earliest organisms is difficult because much remains unknown about the chemical and physical characteristics of the early Earth. Nevertheless, researchers have been developing hypotheses of how self-replicating organisms could form and begin to evolve, and they have tested the plausibility of these hypotheses in laboratories. While none of these hypotheses has yet achieved consensus, some progress has been made on these fundamental questions.

Since the 1950s hundreds of laboratory experiments have shown that Earth’s simplest chemical compounds, including water and volcanic gases, could have reacted to form many of the molecular building blocks of life, including the molecules that make up proteins, DNA, and cell membranes. Meteorites from outer space also contain some of these chemical building blocks, and astronomers using radio telescopes have found many of these molecules in interstellar space.

For life to begin, three conditions had to be met. First, groups of molecules that could reproduce themselves had to come together. Second, copies of these molecular assemblages had to exhibit variation, so that some were better able

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