25 ways the world changed in 2015

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Last year was a year to remember: Three countries ruled in favor of same-sex marriage. We came closer than ever to editing genetically linked diseases out of our DNA. Taking action to prevent gun violence is a bigger priority. And, after years of anticipation, McDonald's finally launched all-day breakfast.

Yes, 2015 was one for the books. It wasn't all good, but the world changed in many ways that will continue to shape our future.

Keep scrolling to see 25 ways the world changed in 2015.

Donald Trump changed the political game.

Trump's inflammatory statements and proposals — to ban Muslims from entering the US, or saying the Mexican government sends its "rapists" across the border — have only served to embolden his supporters. At the end of December, Trump had a sizable lead over other Republican candidates, with 39% of GOP support, according to a nationwide CNN/ORC poll.

The rising support is not due to a lack of reprimand from the media or the public — even members of his own party have condemned his remarks. But unlike other politicians who reverse course and apologize for their blunders, Trump has stuck to his guns and not changed, or apologized for, his opinions. He says exactly what he thinks and feels and this, perhaps, could be his Trump card.

ISIS became a more powerful and deadly terror threat.

Attacks led by affiliates of the terrorist group ISIS in Paris in November that left 130 people dead, as well as the takedown of a Russian airliner in October that killed all 224 people on board, were among a number of "lone-wolf" terror attacks have made ISIS public terror enemy No. 1.

Feelings of alienation and a rift among Sunni and Shiite communities in Iraq and Syria, as well as chaos in Syria and other political turmoil in the Middle East, gave the terrorist group a hand in gaining momentum in 2015, and even in recruiting people from the UK, the US, and other countries.

The US has made fighting Islamic extremist forces like ISIS and other terrorist groups a priority since September 11. And other countries like Egypt, Syria, France, the UK, and Russia are now banding together to dismantle a violent organization.

We came closer than ever to editing human DNA.

This year, scientists used CRISPR/Cas9 — a gene-editing tool that uses a "find-and-replace" technique to replace faulty genes with potentially healthier ones — to modify genes in pigs to make their organs viable for transplants into humans. What sets CRISPR apart from other gene-editing tools is that CRISPR is easier and less expensive to use than tools of the past.

CRISPR has also been used to edit the genes in mice that were genetically predisposed to muscular dystrophy, and to make super-muscular beagles. It has major potential to edit out genetic diseases from humans, and by 2017 scientists will use it to try to treat human adults with a rare form of blindness.

The only potential downside is that editing a gene out of one species could have unintended consequences on another — for example, if we edited mosquito DNA to prevent the species from spreading dengue fever to humans, but accidentally transferred the modified gene to a related species and wiped it out.