I think I can shed light on it. We're well past our prime now, but ~40 yrs ago my wife and I were hiking (and backpacking) with our baby and dog. These days we walk or hike nearly every day with our dog, and this time of year have to take heat and toxic algae into consideration. We live an hour drive north of San Francisco, and 3 or 4 hours from Mariposa. We organized hundreds of hikes and other outings for our club, so we have a lot of experience with hiking, and things going wrong on hikes including the death of a dear friend from a heart attack. I have NEVER hiked up a long, steep climb in that kind of heat. I don't think I've ever done any hiking in that kind of heat. I agree with Runswithdogs comments about the effect of heat on dogs, my dogs have less tolerance than I do. My opinion is that the dog and adults were all suffering severely from the heat and exertion of the climb, but the dog collapsed first. This happened either where they were found or the dog was carried there. JG was older, some photos show him with a bit of a paunch, and he was carrying the baby (and maybe the dog). He was the next to overheat and be unable to go on, either after carrying the dog a short distance or after stopping where the dog collapsed. EC was younger and thinner, and not carrying the baby, so the last to overheat. Leave your husband, baby and dog? No, she would have stayed with them and done what she could as she overheated too, which wouldn't take long. At that point she went for help, but only made it 100 feet/30m before collapsing. I have no experience with extreme heat and babies, and it's just too sad to contemplate, so I'm not going to speculate about that beautiful child. In that kind of heat, and steep slope, there is no "running" to the air conditioned truck. Continuing up that steep slope in that heat was not survivable, staying where they were in that heat with no shade was not survivable, and by then it was too far to go back down to where there was shade. There was no way out once they were in the middle of those switchbacks. My own opinion, based on decades of hiking, and planning and organizing hundreds of hikes.