Suppose World War 3 occurs and manages to wipe out all humans and animals. Is the cycle of death and rebirth broken? Or will human beings be magically reborn again in order to continue their journey to Nirvana?
'Humans' are people who have a moral conscience &amp; are humane.
'Animals' are people driven by instinct (such as survivalism, tribalism &amp; terroritalism), emotion, without reflective wisdom.
'Human' &amp; 'animal' are states of mind rather than physical qualities.
For example, the 'human state' is described as follows:

Sooner, I say, would that blind turtle, coming to the surface once every hundred years, insert its neck into that yoke with a single hole than the fool who has gone once to the nether world would regain the human state. For what reason? Because here, bhikkhus, there is no conduct guided by the Dhamma, no righteous conduct, no wholesome activity, no meritorious activity. Here there prevails mutual devouring, the devouring of the weak. For what reason? Because, bhikkhus, they have not seen the Four Noble Truths. What four? The noble truth of suffering … the noble truth of the way leading to the cessation of suffering.
SN 56.47

AN 6.39 says:

Bhikkhus, a god, a human or any other good state would not be evident from actions born of greed, hate and delusion. Yet, bhikkhus, from actions born of greed, hate and delusion a hellish being, an animal birth a ghostly birth or some other bad state would be evident.
AN 6.39

As for Nirvana, the suttas described it as the here-&amp;-now end of greed, hatred &amp; delusion. There is no 'journey' to 'Nirvana'. To 'journey' is to 'wander &amp; roam'. 'Wandering &amp; roaming' is called 'samsara'. 'Nirvana' is found in the present. 'Samsara' is not 'Nirvana' and 'Nirvana' is not 'samara'.