Christine Hiney I think each book could stand alone. But I agree that you should start with clan of the cave bear. In my opinion The valley of the horses and the mammoth hunters were the best books of the series. But they are all enjoyable. The author does back story for readers that read them out of order. Which helps but you have a better clarity if you read them in order.
Yuri Hinata In my opinion, the whole series is amazing. I think you would enjoy this book quite a lot when you are going to read it, but I also think you would enjoy it way more if you've read the first three books. Some things about the first three books will be explained throughout the book, but other things that happened to the protagonist will not be mentioned, and that might leave you puzzled at why some things are happening the way they are. Also, Auel did a great job on developing the characters in these books, and if you skip the first three books, you'll miss that development. In any case, I'm quite late answering your question, so you might've already read the book(s). If so, I hope you enjoyed it!
Matthew Mccrady Auel tends to repeat information you might need from previous novels, so you could probably start with this book and skip the earlier ones. However, I don't know how rewarding that would be. I would start at the beginning with "Clan."
Kosjenka Quite the opposite in my opinion... first two books are the best, and every subsequent one is a bit worse. This is the 5th sequel, so... if you want books to enjoy, read the first two. In this book you will get a good idea about what happened in the previous books, but you won't get so much pleasure.