Ellis Silver takes a number of features of human physiology, claims they are inexplicable by evolution, and comes to the extreme conclusion that aliens placed humans on Earth.
However, each of the features are actually prosaically explained by evolution.
Bad backs - The Daily Telegraph (A terrible source, I admit, but I only have to do better than the Daily Mail!)
Similarly, when our ancestors began walking on two feet six or seven million years ago, they prompted skeletal changes which today result in bad backs.
Our spines were originally arch-shaped, but standing upright turned the backbone into a weight-bearing pillar, causing the development of the "S" shaped curves which help us balance and walk but also cause lower back pain.
Sunburn - Reddit - ELI5 (Another terrible source, but I am trying to target readers who have very little understanding of evolution, or they would have already worked this out):
This is why people in different regions evolved different skin tones. In Africa and Australia, there is harsh and direct sunlight a majority of the time. People there have very black skin, full of a pigment that blocks out the sun. They don't sunburn, but they find it hard to synthesise Vitamin D... which is okay, because there's so much sunlight shining on them so often that they'll synthesise it anyway, even with reduced efficiency.
The people that migrated far north, to Russia, to Scandinavia, to the United Kingdom and so on, quickly adapted to having very light skin, because their situation was the opposite. There was very little harsh and direct sunlight where they lived, so they shed the black pigment, making them more prone to sunburn (which isn't a concern in Oslo) but more efficient at synthesising Vitamin D (which they needed).
This reveals how deeply racist the claim that "humans" are susceptible to sun-burn is.
In 2015, evolutionary biologist and noted skeptic Dr PZ Myers reviewed the claims in the book:
You are not surprised, I can tell. It’s full of really bad arguments: we are not native to earth because we get sunburns and bad backs, for instance. But essentially every creature on the planet avoids excessive exposure to solar radiation, except the ones that live in caves or the deep sea, and lots of creatures exhibit aging-related disorders — dinosaurs included. Are we to conclude, then, that every species on the planet, from flatworm to elephant, was brought here by aliens?
He goes into further details, pointing out the basic errors in genetics.
I’ve known a few geneticists in my time, and I don’t know of any who wouldn’t double over in laughter at that hypothesis.
Ellis Silver has published a vanity book in which he makes some out-of-this-world claims, which are not supported by evidence. Certainly not enough evidence to discard our existing well-established understanding of human evolution.