The sentence fragment from the last paragraph – living in large groups became untenable – implies that

How climate change ended world’s first great civilisations
David Keys
Monday, 3 March 2014
The world’s first great civilisations appear to have collapsed because of an ancient episode of climate change – according to new research carried out by scientists and archaeologists. Their investigation demonstrates that the Bronze Age ‘megacities’ of the Indus Valley region of Pakistan and north-west India declined during the 21st and 20th centuries BC and never recovered – because of a dramatic increase in drought conditions. The research, carried out by the University of Cambridge and India’s Banaras Hindu University, reveals that a series of droughts lasting some 200 years hit the Indus Valley zone – and was probably responsible for the rapid decline of the great Bronze Age urban civilisation of that region.
It’s now thought likely that the droughts at around that time were partly responsible for the collapse not only of the Indus Valley Civilisation, but also of the ancient Akkadian Empire, Old Kingdom Egypt and possibly Early Bronze Age civilisations in Greece. “Our evidence suggests that it was the most intense period of drought – probably due to frequent monsoon failure – in the 5000 year-long period we have examined,” said University of Cambridge Palaeoclimate scientist Professor David Hodell. The scientists studying the collapse of the Indus Valley Civilisation obtained their new evidence from a dried-up lake bed near India’s capital New Delhi which is just 40 miles east of the eastern edge of the Indus Valley Civilisation.
The Indus Valley ‘megacities’ – some with populations of up to 100,000 – rapidly declined. Populations shrank and the old urban civilisation, which had lasted 500 years, collapsed.
“Archaeologists get an opportunity to investigate how ancient populations responded to climatic and environmental change,” said University of Cambridge archaeologist, Dr. Cameron Petrie. “For the Indus populations, it looks as though living in large groups became untenable, and it was much more sustainable to live in smaller groups. This is of course a huge simplification of a complex process, but this transformation is the underlying dynamicˮ.
(www.independent.co.uk. Adaptado.)

big cities became an indefensible option of life.

life in smaller towns were much more pleasant.

a village provides safer living conditions.

smaller groups can be unsustainable by themselves.

big number of people living together can help each other.