I’ve not as yet encountered a scientific explanation as to how Adam could have been the result of evolution.
Start with an ape similar to a chimpanzee, but not actually a chimpanzee. It lived in a forest. Then around six million years ago the climate changed and the trees began to die back, leaving more savannah grasslands with isolated trees. Some of the apes remained in the forest, others moved out onto the savannah. Evolution adapted the apes who moved to better survive in the savannah, basically bipedalism. They were still good at climbing trees, but if a lion appeared they had to run to the nearest tree before they could climb it. In the jungle the nearest tree was a lot closer. The jungle apes were already adapted to the jungle environment, so did not change much.
That gives a bipedal ape, Australopithecus, living in the savannah. Over time the brain size of these apes increased. Australopithecus’ skull had a capacity of 350 to 600 cc, compared to a chimp at about 300 to 480 cc. The size increased further and about two million years ago a new genus evolved: Homo. Cranial capacity for an early Homo,
Homo habilis was about 650 cc – larger than any Australopithecus.
New species evolved within the Homo genus.
Homo erectus had a cranial capacity of 850 to 1,000 cc. The long term trend towards a larger brain was continuing. Erectus was the first species of Homo to move out of Africa. Some of the first erectus fossils found were in Java and China. Very primitive stone tools (Acheulean) and fire are commonly associated with erectus fossils.
From the widespread erectus population, different groups emerged, for example
Homo neanderthalensis in Europe,
Homo denisova in the Urals and
Homo sapiens in Africa.
Palaeontologists cannot tell which of these various species had souls – souls do not fossilise and do not have a particular DNA sequence. Which specific pair were given souls by God cannot be determined. What can be said is that the single pair, or their offspring, interbred with their biologically compatible unsouled contemporaries to give the range of genetic variation we see in modern humans. God could then have given souls to such of their offspring as He wished. Any descendant of any of Adam and Eve’s children would necessarily be descended from Adam and Eve.
In this context the Pope talks about “true humans”, which I take to mean biological humans with human souls. A biologically compatible, but unsouled member of the same species would be able to physically interbreed, but would not be a “true” human, merely a biological human. Such a mating would be “open to life”, which appears important in Catholic doctrine. God has the power to give the new life a human soul.