This piece originally aired on May 19, 2015.

The Pentagon is ordering all branches of the military to integrate their ground combat units by the end of the year, or explain why women can't do the job. To help answer that, the Marine Corps has turned to some of the toughest female and male Marines out there, who are participating in an experiment that could help decide the future of women in war, reports CBS News correspondent Jan Crawford.

It may look like standard military training, but in the California desert, a group of Marines could be making history.

"It's not necessarily for us, specifically. It's for that one woman, that one female, that joins out there that wants to do this eventually -- that she's given the opportunity to do it," Sgt. Danielle Beck said.

Beck is one of more than 250 Marines who volunteered for a grueling mission: an unprecedented scientific experiment that will help the Pentagon decide whether women can fight on the front lines.

About seven percent of the Marine Corps is now women; many already serve in combat in support positions.

But some jobs, like the hard-charging infantry, remain limited to men

The forefront of battle is "where rubber meets the road," Cpt. Raymond Kaster said.

"Those are the difference makers. Those are the trigger pullers, the infantry Marines," he said. "We are supported by a cast of amazing Marines that make us able to do our job. But the infantry is where we win battles."

Infantry Marines are trained to fight in close combat -- to search out and destroy the enemy.

The question is whether it should be opened up to women, and that's what the group of Marines at 29 Palms will help determine. .

They made it through intense specialized training for months of simulated battle, hikes with more than 100 pounds of equipment, heavy weapons gunfights with live ammunition and working as a unit to overcome obstacles.

Throughout, researchers watch and record their performance.

Each Marine is equipped with high-tech GPS sensors to measure accuracy and heart rate monitors to see how their bodies react.

"You wanna turn the gender lens off for a moment and just look at the physical characteristics and see, you know, maybe it's height, maybe it's upper body strength, maybe it's lower body strength. Maybe it's your heart efficiency. What physical characteristics really lead you to be good in these jobs," said scientist Paul Johnson who helped develop the testing.

The data will help the Marines decide what impact women would have on combat units, and develop new physical standards that apply to men and women.

"They're all capable of performing the tasks. The question is can they do them to the same level as their male counterparts who are doing 'em now," Johnson said.

It's as close to war as you can get. Johnson is testing the Marines in situations they could actually encounter.

Using a dummy called Corporal Cal, participants simulate rescuing a fallen comrade at the end of an exhausting battle. Carl weighs 174 pounds, the weight of the average Marine.

"They have to get Corporal Carl to some kind of medevac point," Johnson said.

Critics of women in combat have said it could disrupt the cohesion of the unit. But these Marines say that is not an issue.

"We all know how to work together. They go hand in hand: the training together as well as being able to pass those physical standards. And once you have those two together there should be no reason to not trust the Marine to the right or left, whether they be male or female," Cpl. John Wood said.

"I'm just going to look at it, not only do I have my brother in arms but I have my sister in arms that I have to protect as well," Cpl .Tevin Guion added.

For the women, the physical part has been the challenge. They are smaller and on average about 30 pounds lighter than men. But in combat they have to be equal. They have to carry the same load and be just as fast as the men.

"It's not that we can't carry weight. We can carry the weight. It's the pace especially when we match up with the males. You're looking at our size and we have males that are almost 6 foot with longer strides and it's hard to catch up with them, keep up with them," Cpl. Janelle Lopez said.

They do not think the Marines should lower standards for women. On that point, there is wide agreement.

"You have to perform. If you can't show up ready to go physically and mentally, we don't just lose a game. We don't just, you know, not make a sale. Lives are at stake. That's why this is so important to the infantry community and the Marine Corps as a whole for the next 20, 30 years," Kaster said.

The women say few would want these roles, but they're doing it so those few will have the chance.

"It takes a special person, a special drive and a special mindset and a special heart to do this. There's not many of us. And it goes the same way for the men," Beck said.

"It's just a sense of pride. I started this. I'm def going to try to finish. I'm not going to quit just because it's hard," Lopez added.