How come the Borg could not adapt to Species 8472? Voyager was able to create nano probes that could destroy Species 8472 and yet the Borg couldn't figure that out? Why?
From Memory Alpha:
[T]he Borg learned about different species solely by means of assimilation; they were unable to understand or adapt to Species 8472's technology. This organically based technology was more than a match for Borg defenses. Species 8472's vessels were nearly impervious to all types of conventional weaponry, while their weapons were able to penetrate Borg shields with ease.
The issue is that the Borg didn't know a solution to how to assimilate 8472, and they grow their knowledge through assimilation, not experimentation and creativity like humans.
The borg adapt in several ways:
- grab someone who knows how to defeat technology X
- examine technology X to find a known pattern
- In the case of adapting to phasers, it's just a matter of figuring out the frequency shift patterns and adjusting shields to match it.
- in the case of cloaks, etc, it's a matter of observing it happen, and then being able to process fast enough to spot the small but important telltales.
- try semi-random adaptation attempts from the standard library until something works.
In the case of 8472, they hadn't found the needed combination yet, and appear to have been trying approach 3.
- Humans make intuitive "fill-in-the-gaps" while borg don't.
- Humans tend to qualitatively judge while borg seem to boolean judge things; this allows humans to better detect similarity of a failure that took longer to fail and of success.
- Humans thus could see what partially worked as a partial success, and go from there, while the borg simply chalk that attempt as a failure.
- until the borg have failed in all major trees, they are unlikely to start looking at various branches of failed tree stems; given their history, they have a LOT of stems to try before they get to additional branches of stems where the stem didn't succeed.
- due to 8472's successes versus the borg, reports were limited as to what had been tried, leading to duplication of failed attempts.
When the Borg attempted to assimilate Species 8472, they found that the species was immune to assimilation -- their immune system was so efficient that any foreign body their cells encountered was immediately destroyed, including Borg nanoprobes.
Moreover, since the Borg learned about different species solely by means of assimilation; they were unable to understand or adapt to Species 8472's technology.
from memory alpha
They actually mentioned this in Scorpion 1 I believe. Janeway mentioned in the briefing that the Borg learn via assimilation and what they can't assimilate, they can't understand. The Voyager crew however study things to learn. They look at the data they've collected and figure out the best possible actions by trial and error and testing.