Ch 25 (1)

The sweeping changes inlife on Earth revealed by fossils illustrate __, the pattern of evolution over large time scales.

Experinments in chem, geology, and physics have led scientists to propose one scenario: they hypothesize that chem and phys processes on early Earth, aided by the emerging force of natural selection could have produced very simple cells through a sequence of four main stages: __

True or False:
For the first few hundred million years, life prob couldn't have originated or survived on Eaarth because the planet was still being bombarded by huge chunks of rock and ice left over from formation of the solar system.
True False

True or False: The first atmosphere was probably thick with water vaapor, along with various compounds releasead by volcanic eruptions, including nitrogen and its oxides, carbon dioxide, methane, ammonia, hydrogen and hydrogen sulfide. As Earth cooled, the water vapor condensed into oceans, and much of the hydrogen quickly escaped into space.
True False

In teh 1920s , scientists Oparin and Haldane hypothesized that Earth's early atmosphere was a __ (electron-adding) environment, in which organic compounds could have formed from simple molecules. The energy for this organic synthesis could have come from __ and intense __. Haldane suggested that the early oceans were a solution of organic moleucles, a "__" from which life arose.

It is unclear whether the atmosphere of young Earth containedd enough __ and __ to be reducing. Growing evidence suggests that the early atmosphere was made up primarily of __ and __ and was neither reducing nore oxidizing (electron removing). It is possible that small "pockets" of the early atmosphere- like volcanic openings- were reducing. Perhaps instead of forming in the atmosphere, the first organic compounds formed near submerged volcanoes and deep sea vents, where hot water and minerals gush into the ocean from Earth's interior. These regions are also rich in inorganic sulfur and iron compounds, which are important in __ synthesis by present-day organisms.

The presence of small organic molecules, like __, is not sufficient for the emergence of life as we know it. Every cell has a vast assortment of __, including enzymes and other proteins and the nucleic acids that are essential for self-replication. The __ formed spontaneously, w/o the help of enzymes or ribosomes. But unlike proteins, these __ are a complex mix of linked and cross-linked amino acids. Nevertheless, it is posible that such polymers may have acted as weak catalysts for a variety of reactions on early Earth.

Two key properties of life are __ and __. Neither property can exist without the other. DNA molecules carry genetic info, including the instructions needed to replicate themselves accurately. But the repiclation of DNA requires elaborate enzymatic machinery, along with a copious suppy of nucleotide builiding blocks that must be provided by the cell's metabolism.

If building blocks of __ were not part of the early organic soup, slef-replicating molecules and a metabolism-like source of the building blocks must have appeared together. How did that happen?
o The necessary conditions may have been met by __, collections of abiotically produced molecules surrounded by a membrane-like structure. __ may exhibit some properties of life, including __ and __,as well as the maintenance of an internal chemical environment different from that of their surroundings.

Lab experiments demonstrate that __ could have formed spontaneously from abiotically produced organic compounds. For example, certain small membrane-bound droplets called __ can form when lipids or other organic molecules are added to water. The hydrophobic molecules in the mixture organize into a bilayer at the surface of the droplet, much like the lipid bilayer of a plasma membrane. __ can "reproduce" and because their bilayer is selectively permeable, __ undergo osmotic swelling or shrinking when placed in solutions of different solute concentrations. Some of these __ can perform simple metabolic reactions, another importnt step towrd the origin of life.

The first genetic material was most likely __, not __. Scientists found that __, which palys a central role in protein synthesis, can also carry out a number of enzyme-like catalytic functions. Cech called tehse RNA catalysts __; some can make complementary copies of short pieces of __, provided that they are supplied with nucleotide building blocks.

__ on teh molecular level has produced __ capable of self-replication in the lab. Unlike double-tranded DNA, which takes the form of a uniform helix, single-stranded __ molecules assume a variety of specific 3D shapes mandated by their nucleotide sequences. In a part. environment, RNA molecules with certain base sequences are more stable and replicate faster and with fewer errors than other sequences. The RNA moleucle whose sequence is best suited to the surrounding environment and has the greatest ability to repplicate itself will leave the most descendant molecuels. Its descendants will not be a single RNA "__" but instead will be a family of sequences that differ slightly because of copying errors.

Occasionally, a copying error will result in a molecule that folds into a shape into a shape that is even more stable or more adept at __ than the ancestral sequence. Similar __ events may have occurred on Early Earth. Thus, the __ of today may have been preceded by an "__ world" in which small __ moleucles that carried genetic info were abe to replicate and to store information about the __ that carried them.

A __ with self-replicating, catalytic RNA would differ from its many neighbors that did not carry RNA or that carried RNA without such capabilities. If that __ could grow, split and pass its RNA molecules to its daughters , the daughters would have some of the properties of their parent. Although the first such __ must have carried only likited amounts of genetic info, specifying only a few properties, their inherited characteristics could have been acted on by __. The most successful of the early __would have increased in number because they could exploit tehir resources effectively and pass their abilities on to subsequent generations. This emergence of such __ may seem unlikely, but remember that there could have been trillions of __ in bodies of water on early Earth. Even those with only a limited capacity for inheritance would have had a huge advantage over the rest.

Once __ that carried genetic info appeared in __, many further changes would have been possible. For example, RNA could have provided the template on which DNA nucleotides were assembled. Double-stranded DNA is a much more stable repository for genetic info than the more fragile single-stranded __. __ also can be replciated more accurately.

Accurate replication was a necessity as __ grew larger through gene duplication and other processes and as more properties of the __ became coded in genetic info. After DNA appeared, perhaps RNA molecules began to take on their rpresent-day roles as __ in the translation of genetic programs, and the RNA world gave way to a __. The stage was not set for a blossoming of diverse-life forms- a change we see documented in the __.