Re: how long have spiders existed and what did they originate from

Spiders are not my specialty, but I'll give you the best answer that I can.

Spiders are small creatures with few hard body parts that might fossilize. 
This means that finding a spider fossil is extremely hard, and that there 
are probably many gaps in our understanding of their evolution.

Spiders are members of a group called Cheliceramorpha (animals with claws 
at the ends of their limbs) [technically this group is a "branch" of the 
"phylum" arthropoda, which also includes insects and crabs]. Members of 
this group are found among the earliest fossilized animals, dating back to 
the Cambrian Period (600 million to 500 million years ago). These animals 
were all aquatic creatures. Although there were many spider-like creatures 
living during this time, the first true spiders did not appear until later.

Spiders themselves belong to a more specific group, the class Arachnida, 
which includes scorpions, mites and spiders. This group first appeared 
during the Silurian Period (440 million to 400 million years ago), and were 
among the first groups of animals to inhabit dry land. Once this group 
established itself, it diversified into many other types of animals. Among 
these "other" types were the first recognizable spiders, which appeared in 
the fossil record during the Pennsylvanian Period (310 million to 270 
million years ago).

So, spiders that we would recognized as spiders have been around for about 
300 million years. Things that we might think of as spiders, but that 
really aren't spiders, have been around for almost 600 million years. By 
way of comparison, our species (Homo sapiens) has been around for only 
300,000 years, and our group (the class Hominoidea) has been around for 
about 20 million years.

More information about spiders and their evolution can be found at: 
http://www.ucmp.berkeley.edu/arthropoda/chelicerata/chelicerafr.html