为什么我在线程中不能访问一些库类?

为什么要这样做

require "bio"

threads = (1..2).map do
  Thread.new do
    seqs = ["gattaca"] * 5
    alignment = Bio::Alignment.new(seqs)
  end
end

threads.each {|th| th.join} ; nil

给出此错误消息?

NameError: uninitialized constant Bio::Alignment
    from (irb):6
    from (irb):10:in `join'
    from (irb):10
    from (irb):10:in `each'
    from (irb):10