You say a common ancestry is a huge assumption that cannot be validated.
Retroviruses reproduce by inserting their genetic material into a cell at random, which gives the cell a code for reproducing the virus. In some infected cells the virus ends up in a stretch of DNA that is ignored by the inner workings of a cell. So the virus has no effect and if the cell happens to be a germ line cell, the retrovirus sequence is inherited as "junk DNA", basically forever. There are called inactivated endogenous retroviruses and they are very common in all genomes, There are thousands of different retroviruses. Some cause AIDS, others give you a cold.
About ten years ago the entire human DNA sequence was published. There are about 3 billion base pairs of DNA in the human genome. The chimpanzee genome was published in 2005. It has 3 billion base pairs too. It was reported that there were seven different retrovirus sequences in exactly corresponding positions in the chimpanzee and human genomes.
Either these seven different retroviruses just happened to infect humans and chimps in exactly the same places in strings of 3 billion base pairs, in a germ cell, and were inactivated because they had landed in the wrong spots. The probability of this has been calculated as one in 10 to the 80th power. The alternative explanation is that they infected germ line cells of common ancestors.
You say formation of a new species has never been observed. A biological species may be distinguished from another very similar one by the fact that they usually refuse to mate, or that if they mate they do not produce fertile offspring. Thus horses and donkeys are different species since their progeny is almost always sterile.
In North America, the European apple was introduced about 1605. A related tree called the North American hawthorn was infested by an insect called the maggot fly. This lays its eggs on the hawthorn at a time related to the tree flowering and setting fruit. The apple flowers and sets fruit at a different time, so if the fly laid eggs on an apple the larvae would be expected to starve.
About 1860 a new pest of apples appeared in north American orchards. This looked the same as the hawthorn maggot fly but it laid eggs at a different time. These insects have been studied for obvious reasons. Attempts to cross breed the apple and hawthorn varieties fail about 19 times out of 20, showing that the two varieties have almost formed different species since 1605. In another few hundred years they will be different species and will not crossbreed at all.
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Until about 1995 I imagined that anti-evolutionists were innocently wrong but made a few good points.
Then I was handed a book published by a religious organisation which claimed to disprove evolution. By the time I had read the first two pages I was beginning to think something was wrong. Using a common dictionary and a one-volume encyclopedia I checked some of the claims in the book. Nearly all of them were wrong and some of them looked like direct lies, because they directly contradicted the very simple stuff in the encyclopedia.
A year or so later I read a book on the matter, written by a geologist in 1994 and with a foreword by a very senior churchman, the top of his sect in this country. This book went through many of the claims of anti-evolutionists and refuted them. It also exposed a well known American creationist as a deliberate liar, showed that many of these creationist leaders had faked or very low grade academic or scientific qualifications and that some had been investigated by police for financial fraud. That was not all. It showed how at least two of these organisation had been hoaxed by simple stuff that if they had investigated, they would not have published in magazines and a TV show about Noah's Ark. Yet they claimed to be "scientists".
The patent office where I worked was reorganised about 1996 and our section was landed with a lot of material we had not handled before, so we had to read it pretty deeply. It mostly concerned medicine directly, but indirectly the evidence in it showed exactly how biological evolution happened on a molecular scale. There were long lists of protein and DNA sequences, detailing slight changes from one to the other in humans and also in animals. It was our job to confirm these changes were actually there. These are the famous mutations and they are real.
There are thousands of documents like that. Most of them were prepared by organisations with big research laboratories, mostly drug companies.These companies would not be spending millions on research and getting patents in a dozen countries if the material in the applications was not genuine.
I have looked at the web sites of some creationist organisations. I find the same material on them that was refuted in 1994.