(Answer to question 1 ) All living things are related and have descended with modification from organisms that lived in the
past. The mechanism is called natural selection, which states that the organisms that are best adapted
to their environment are the most likely to survive to reproduce, causing species to evolve with time.
(Answer to question 2 ) Interactions between plates determine, to some extent, which of the three rock groups will form. Plates
may melt to form igneous rocks, be altered to form metamorphic, or sink to allow sediments to become
sedimentary rocks.
(Answer to question 3 ) The three concentric layers are determined by density. At Earths center is the most dense layer, the
core, with a calculated density of 10-13 grams per cubic centimeter. At 16 of the total volume, the
core is mostly iron with a small amount of nickel. There are two layers, a solid interior and a liquid
outer layer. The mantle makes up most of the planets volume, 83. The mantle is much less dense
than the core, 3.3-5.7 grams per centimeter cubed. It is composed mostly of dark, dense peridotite. The
outermost layer is the least dense layer, the crust. There are two major types, continental crust, which
is thick and has an average density of 2.7 grams per cubic centimeter and oceanic crust, which is
thinner and density at 3.0 grams per cubic centimeter.
(Answer to question 4 ) In the solar nebula, gases and solid particles condensed in localized eddies and then accreted into
planetesimals. These bodies collided and grew until they became planets. The planets nearer the Sun
were small and composed of rock and metallic elements. One of these was Earth. Heat was generated
by meteorite impacts, gravitational compression, and nuclear reactions. The heat caused the material to
melt. Since it could now move, dense metals sank to the core and denser silicates to the mantle.
Lighter silicates rose to form the crust.
(Answer to question 5 ) Scientists believe that the universe originated with the Big Bang approximately 14 billion years ago.
At the moment of the Big Bang, there was only energy, no matter. In the first second following the Big
Bang, the four basic forces separated, and the universe experienced enormous expansion. Galaxies and
stars came together, and the chemical makeup of the universe changed. In the early universe, the only
chemical elements were hydrogen and helium, but now 2 of the universe is all other elements by
weight.
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