One thousand years ago, when the United States of America did not exist and Oxford and Cambridge were backwaters of ignorance, the light of human reason shone brightly in places like Tunis, Cairo, and Baghdad. During the Abbasid caliphate for much of the 8th through middle 11th centuries, and also sporadically thereafter, tolerance of certain non-Muslim groups was enshrined in law. This was not as extensive as the constitutionally guaranteed religious (and non-religious) freedoms we enjoy in the West today, but it did mean that non-Muslims such as Musa Ibn Maimun (also known as Maimonides), Hunayn ibn Ishaq, and Yuhanna Ibn Bukhtishu, could not only practice their Judaism or Christianity, but could also make enduring contributions to the social and intellectual life of the then-dominant Muslim culture.
It may not be a coincidence that many aspects of our understanding of the world have roots in this age. Arab and Persian scholars (Muslim and non-Muslim alike) not only translated the writings of the Greeks, but also made original contributions about mathematics, medicine, and social science (among other topics). Regarding biology, one of the more interesting claims that surfaces from time to time concerns evolution:
The theory of biological evolution in its complete form was presented by a great early zoologist, al-Jahiz in the ninth century.
Not quite. There are indeed some tantalizing quotes that evoke evolutionary ideas from medieval Arabic scholars. The Englishman John William Draper famously referred to "The Mohammedan Theory of Evolution" in his 1874 book History of the Conflict Between Religion and Science, probably in reference to the Arabic scholar Ibn Khaldun, who wrote in his 14th century The Muqaddimah,
creation ... started out from the minerals and progressed, in an ingenious, gradual manner, to plants and animals. The last stage of minerals is connected with the first stage of plants, such as herbs and seedless plants. The last stage of plants, such as palms and vines, is connected with the first stage of animals, such as snails and shellfish ... the last stage of each group is fully prepared to become the first stage of the next group.
In sum, no animal can survive without nourishment. The hunting animal cannot escape being hunted. Every weak animal devours those that are weaker; every strong animal cannot avoid being consumed by those that are stronger.... God, in sum, made some beings the cause of life to others, and in turn made these the cause of death to yet others.
These two observations--- continuity of existence across different forms of life (and indeed non-life) from Ibn Khaldun, and a struggle for existence among individuals from al-Jahiz--- are indeed relevant to the contemporary theory of biological evolution. However, this is a far cry from "biological evolution in its complete form". First of all, the goal of al-Jahiz's Book of Animals was, according to the German historian Herbert Eisenstein, "not actually the study of animal species, but a proof of the existence of the Creator that is evident from his creation" (p. 122 in Einfürung in die arabische Zoographie). Moreover, al-Jahiz was a gifted philosopher and theologian interested in biology, not vice-versa. He saw free will and the autonomy of God's creation as the reason why animals were "created" with the means (e.g., claws, fangs, spines) to attack others and defend themselves. According to Einsenstein (continuing the passage quoted above), al-Jahiz wrote that
when you see an animal ... of great danger, and concerning whom Man must be very careful, such as snakes and wolves provided with fangs ... thus may you know ... that God--- sublime and powerful is He--- gives to the steadfast, those who understand that free will and rational experience could not exist if the world were purely evil or entirely good.
Whether or not you think this argument is convincing (and it remains a key part of modern religion's approach to theodicy), the point is that al-Jahiz was less interested in the natural mechanisms by which life became diverse over time than he was in understanding nature in the context of monotheistic philosophy.
Another difference between modern biology and the natural philosophy of the Arab-Persian golden age was, ironically, the reality of the species. The notion that a group of interbreeding animals is thereby distinct from other such groups is not evident in pre-modern Arabic literature. Al-Jahiz was fascinated with hybridization and adaptation, and contemplated if and how environmental factors may have had an influence across generations of humans and other animals. According to another German translation of a passage from al-Jahiz, he wrote that maybe eels arose by cross-breeding fish with snakes. Maybe people from the western Caliphate (what is today Morocco and Algeria) look different from Arabians because of differences in the food and air. These were the kinds of discussions one can find in the prolific writings of al-Jahiz, Ibn Qutaiba, and others of their time. Were they more informed about nature than Europeans in the 9th century? Yes. Did ninth-century Arabs discover biological evolution? No.
As summarized by the Iraqi-British physicist Jim Al-Khalili in his book Pathfinders (and echoed by writers such as James Montgomery and Rebecca Stott), al-Jahiz articulated a mix of natural theology with Lamarckian evolution, in which behaviors and environmental influences of a given animal could have a direct influence on its offspring. This was more than just quoting Aristotle, and Europeans didn't get this far until many centuries later. However, although the Abbasid caliphate played a key role in upholding and advancing human reason, no one there or anywhere else during the middle ages discovered the major principles of contemporary evolutionary biology.
This well-corroborated theory about the mechanisms behind biological diversity recognizes that, in general, what a given animal does over the course of its lifetime (stretch its neck to reach leaves in the treetops, exercise its arms hammering horseshoes) does not directly influence its offspring. Yes, epigenetics and lateral transfer can make detours around the barrier of generational inheritance, but the essence of evolution is about variety, heritability, selection, and constraint. Evolutionary biology is not a static discipline, and in years to come there will be many more discoveries that shake up our understanding of how the mechanism behind biodiversity works. One such recent discovery is that physical laws underpinning cell growth may have a far stronger influence on certain aspects of anatomy than adaptive selection, as demonstrated in 2013 by a study of scale development in crocodiles. This discovery, and many others over the past 150 years, improves (not disproves) our understanding of evolution.
Eager proponents of today's monotheisms sometimes appear very keen to see in their cultural history and holy books indications of modern scientific discoveries. However, it seems to me that the real take-home message of golden age of the Middle East is not its contributions to modern science (and there are many), but the fact that some of its rulers valued ideas at least as much as ideology. Give scholars the resources to be able to study and write, and encourage curiosity about traditions besides your own. Scientific and civil advancements will happen (and have happened) for the same reasons in the 21st century as they did in the ninth.