"Defending Eden: Book Five, the Darwin's World Series: An Epic of Survival" by Jack L Knapp

"Defending Eden: Book Five, the Darwin's World Series: An Epic of Survival" by Jack L Knapp
  https://www.amazon.com/Defending-Eden-Darwins-World-Survival/dp/107402933X/
Book number five of a five book science fiction time travel parallel universe travel series.  I read the well printed and well bound POD (print on demand) trade paperback published in 2019 by the author that I bought new on Amazon.  I think that the series is finished as the author wrapped up all of his dangling threads in this expansive book.
In the 25th century, humanity has solved all problems and even created machines for time travel and parallel universe travel.  There is no want for anything with unlimited power from fusion reactors and matter conversion units.  But, they caused a new problem, humanity is dying out as people have lost the will to live.
So the future scientists are bringing forward dying people from the 20th century, restoring their bodies to their 20 year old age, and transferring them to Earth 4428, a human less parallel world going through the end of the Pleistocene ice age.  With nothing but a few tools and the clothes on their backs.  Survive or die in the primitive conditions of what will be the southern USA but there are lions, big cats, mammoths, bison, dire wolves, deer, elk, short face bears, grizzlies, etc.  And chest deep snow in the winters.
Matt and several others were deposited by the futurists into what will be the eastern portion of Texas.  They moved to the western side of Texas and closer to the Gulf of Mexico to reduce the number of slaver attackers and the terrible winters.  They settled in what is the Rio Grande area, close to the Gulf of Mexico.  Now the futurists are coming back to Earth 4428 and wanting some of the transplanted survivors to start moving back to Earth Prime.  And another parallel Earth has discovered jumping to parallel universes and is mining unoccupied parallel Earths for needed goods for their 40 billion inhabitants.
Now some of the parallel Earths are being attacked by dogfaced aliens. And pushing them out of their worlds are the ravenous race of froggies who can destroy a world in 20 years by overpopulation.  They are coming for Earth 19 in a 3,000 space ship invasion force.  The froggie invasions of Earth Prime and the other Earths cannot be far behind.
If you liked Robert Heinlein's "Tunnel In The Sky" book, you will probably like this series.
My rating:  5 out of 5 stars
Amazon rating:  4.2 out of 5 stars (147 reviews)
Lynn