Friends:

Breakthrough to the Truths Safeguards Against Downfall Rebirth!

The Blessed Gotama Buddha once explained:
Bhikkhus & friends, see this little bit of dust, I have taken up upon the nail of
my little-finger?  What do you think is most: This tiny bit of dust or this great
Planet Earth? The Bhikkhus then responded:  Venerable Sir, this great Planet Earth
is much more, incomparably more.  The minute speck of dust is trifling, microscopic,
negligible in comparison.

The Blessed Gotama Buddha then pointed out:
Similarly and exactly so too, Bhikkhus & friends, those beings are few and
quite rare, who are reborn among human beings!  Beings reborn
elsewhere, lower, as non-humans are much more numerous and common.
Why is it so?  Because, Bhikkhus, they have not seen the 4 Noble Truths!
What four?
This is Suffering;
Craving is the Cause of Suffering;
No Craving is the End of Suffering;
The Noble 8-fold Way Ends all Suffering.
Therefore, Bhikkhus & friends, exertion should be made Now to understand:
All This is Suffering; This Greedy Craving is the sole Cause of all Suffering;
No Craving is the End of Suffering; The Noble 8-fold Way Ends all Suffering.
Therefore should effort to fathom these 4 Noble Truths be made NOW!

Comments:
Animals are much more common than humans (trillions of billions of insects & fish),
since this rebirth at this level is much more common!  Similarly with ghosts,
demons, and hell beings.  Downfall to these states are common, because evil
and immoral action such as Killing, Stealing, Lying, & Abusing are common!
Ethics is thus the core factor of creation & conditioning of future being.

Source (edited extract):
The Grouped Sayings of the Buddha. Samyutta Nikāya. [V:465-6]
section 56: Saccasamyutta. Thread 61: The Downfall ...

Rare is Human Rebirth!