Chapter 11
Chapter 11: A Biography of Earth
Feature Articles
The Rest of the Story: The Moon: A Source of Insight into the Hadean Earth?
by Stephen Marshak
Our Moon originated soon after the formation of the Earth itself, in the wake of a collision between a Mars-sized body and the Earth (see chapter 1). Upon collision, the impacting body disintegrated; most of its core probably sank into the Earth to combine with Earth’s core, while its mantle, along with fragments of the Earth’s mantle, formed a ring of dust encircling the Earth. The Moon condensed out of this ring. Today (or tonight), even with the unaided eye, you can see two types of surface provinces on the Moon: light-colored lunar highlands that consist of a type of ultramafic igneous rock called anorthosite, and the dark-colored mare (from the Latin word for “sea”) that consist of mafic igneous rock (basalt). Most of the anorthosite solidified before 4.35 Ga, while most of the basalt formed later, between 3.9 and 3.1 Ga.
Rocks at the Moon’s surface have not passed through the rock cycle, for plate tectonics does not occur on the Moon, and the Moon does not have an ocean or atmosphere. Thus, we still can see relicts of the Moon’s earliest history. From these relicts, geologists deduce that soon after it formed, the surface of the Moon was a magma ocean. Direct freezing of this ocean produced the anorthosite of the lunar high-lands. The basalt compromising the mare developed later, when huge meteor impacts pulverized much of the surface rock, creating a thick coating of dust and debris and excavating deep craters. Decompression beneath the craters produced mafic (basaltic) magma that rose to fill them. If this explanation for Moon-rock formation is correct, then the present surface of the Moon provides examples of the types of material that would have formed on the surface of Earth in the Hadean eon, had it frozen. But because the rock cycle does operate on Earth, its surface features have changed radically since the Hadean time.
Why is the Moon so dead? Because of its small size, the Moon has cooled more than has the Earth, and the thickness of its lithosphere accounts for almost 60% of the thickness of the whole planet. Such a thick lithosphere simply cannot break into moving plates.
We can get a sense of the frequency of meteor impact during the Hadean eon by observing the intense crating of the Moon’s surface, for the craters have not eroded away. Considering the relative lack of craters in the mare, it seems that most cratering occurred before about 3.9 Ga, by which time, apparently, most debris in the solar system had been incorporated into larger planets. The pummeling of the Earth and Moon might have been worse were it not for the gravitational attraction of giant planets like Jupiter and Saturn, which sucked in much of the debris in the solar system. Notably, particularly large impacts probably changed both the rotation rate and the tilt of the Earth.
The Rest of the Story: The Cradle of Life
by Stephen Marshak
Most questions concerning how life began on Earth remain unanswered today. But geological studies have provided some important insight into the process. Life forms consist of specialized organic chemicals organized into cells. Laboratory experiments show that amino acids, the building blocks of these chemicals, can be synthesized out of inorganic chemicals from elements that naturally exist at the surface of the Earth. But where did this process take place? Some scientists speculate that amino acids were created when lightning struck shallow seawater ponds that contained the appropriate raw materials. But even in the simplest amino acids cannot survive long in the presence of oxygen, for they will react with oxygen and fall apart. We now know that although the Archean atmosphere held very little oxygen, it was not totally oxygen-free; water molecules hit by ultraviolet radiation in the atmosphere broke down and supplied a trace of oxygen. Thus, amino acids could not have formed in warm ponds at the surface of the Earth.
In the 1970s, when geologists began using submarines to study Earth’s mid-ocean ridge system, they found hot-water vents (black smokers) along the ridge axis (see Chapter 4). Bacteria have colonized the oxygen-free water around these vents. These organisms draw their life-giving energy from the chemical bonds in sulfide minerals that spew out of the vents. Thus, their metabolism differs fundamentally from that of familiar animals and plants, who draw energy from organic chemicals like sugar and starch: the burning of sugar and starch requires oxygen, while the burning of sulfide minerals does not. Considering this discovery, many geologists now speculate that the chemical components of life first formed somewhere in the hot water along the tens of thousands of kilometers of Archean mid-ocean ridges. The first living organisms were likely sulfide-eating bacteria.
The Rest of the Story: Clues in the Fossil Record
by Elizabeth Lane Mason
Paleontologists have the unenviable task of trying to piece together the history of evolution using tiny bits of information separated by huge gaps of time. This effort is particularly difficult in the Precambrian where the fossil record is exceptionally sparse. Mysteriously following this scarcity, almost all the main types of animals (or phyla) that exist today suddenly appear in the fossil record at the beginning of the Cambrian, around 545 million years ago.
Many paleontologists concluded that an explosion of evolutionary activity during the early Cambrian must be responsible. But others aren’t convinced that evolution that rapid is possible. They have suggested a longer period of evolution that left no fossil record must have preceded the Cambrian.
German and British researchers recently made a discovery that lends support to the idea that the Cambrian explosion might not have been that explosive after all, and may have been preceded by a long evolutionary fuse in the Precambrian. They found some of the oldest known crustaceans in Lower Cambrian limestone deposits in Shropshire, England. The 511-year-old fossils are very well preserved with some of the soft body parts cast in calcium phosphate, allowing them to be identified with confidence unlike other rare fossils of this age.
Crustaceans such as these, as well as modern animals like crabs, lobsters and shrimp, are members of the arthropod phylum. Previously, the oldest undoubted crustaceans were from the late Cambrian, which left 40 million years for them to evolve from a primitive arthropod at the beginning of the Cambrian. But the new discovery of early Cambrian crustaceans suggests the process must have started sooner to allow enough time for all the necessary evolutionary steps from arthropod to crustacean.
Though the newly discovered fossils are good evidence for Precambrian evolution, paleontologists would like to find an early arthropod fossil that can be identified with confidence. The search is on, but uncovering such a fossil is akin to finding the proverbial needle in a haystack. The ancestral arthropods would most certainly have been very small and lacking a shell or skeleton that could be preserved for discovery by scientists millions of years later. Still, soft animal parts may be phosphatized as in the case of the recently discovered crustaceans. In spite of the odds mounted against them, fossil hunters continue their search in hopes of finding a Precambrian piece to the evolutionary puzzle.
REFERENCE:
Siveter, D.J., M. Williams, D. Waloszek, 2001. A phosphatocopid crustacean with appendages from the Lower Cambrian. Science 293: 479-481.