19. The History of ALH 84001 and of Life on Earth
This graph shows the times of important events in the development of life on Earth, as well as events in the history of the ALH 84001 martian meteorite. All the ages here are in years before the present, with “Ga” meaning one billion years (1,000,000,000 years). These ages were all determined using the decay rates of radioactive elements.
The solar system, the Sun, and the planets all formed about 4.56 billion years ago. Almost all meteorites date from this time also, except most martian meteorites, which are much younger. The oldest rocks known on Earth formed about 4.0 billion years ago — we think that the Earth had a solid surface, and even oceans, earlier than that, but active geological processes like erosion and plate tectonics have removed all the older rocks. Surprisingly, the martian meteorite ALH 84001 solidified first from molten lava 4.5 billion years ago, so it is older than any known Earth rock.
The oldest evidence of life on Earth is almost as old as the oldest rocks; there are chemical and isotopic clues that some life was present as long ago as 3.85 billion years. The oldest certain evidence of life on Earth, fossil bacteria, is a bit younger at “only” 3.5 billion years old (see slide #27). These bacteria were probably like blue-green algae that still grow today. The possible fossils in ALH 84001 also may have formed at about this time (see slides #23–#26).
The next milestone in the development of life on Earth was the appearance of complex cells, eucaryotes. A eucaryote cell contains a nucleus that encloses its genetic material and many smaller organs that help the cell live. Bacteria are much simpler because they do not have these smaller cell organs.
Next in the development of life on Earth was the origin of creatures with many cells and different types of cells living and working together. The first evidence of these multicellular animals comes from rocks about 650 million years old (or 0.65 billion years old). These fossil animals seem to be quite different from the familiar animals alive today. But by 550 million years ago familiar animals (like clams, sponges, shrimplike animals, and trilobites) were abundant in the oceans.
Animals like humans (hominids) have only lived on Earth for the last few million years. Somewhat before that, about 17 million years ago, the meteorite ALH 84001 was blasted off Mars by an asteroid impact. ALH 84001 finally landed on Earth about 13,000 years ago, near the beginnings of modern civilizations.
Lunar and Planetary Institute