Is there any evidence that suggests that human memory was stronger before we were able to store detailed information externally in the forms of writing or books?
Human beings today have fantastic memories.
Let me sing you Bruce Springsteen's entire discography. Give me a blank globe; I can name all the countries and their capitals. What do you want to know about dog behavior? And so forth. You surely have your own topics, but you just as surely know them as well as I know Star Wars trivia.
What is true, however, is that some cultures place a much higher value on the training of memory.
The European Middle Ages, for example, always had a literate elite, and the role of bureaucracies and written documentation increased significantly in its last centuries. However, the astronomical price of books even for university masters as well as the ultimately oral purpose of education (preaching) and means of instruction (spoken disputations) placed a high premium on memory even for those who could read. For example, sitting in your cell to comment on Peter Lombard's Sententiae--a quite basic academic exercise--you had to bring to bear everything you could from Augustine, the Bible, Aristotle, your teacher. But you couldn't type "biblegateway 1 cor 13" or "bible talks abouts pistachios what verse" into Google. You couldn't go to the library and walk out with City of God and Confessions and Against the Manichees. You had to know them. This is very evident reading medieval texts--there are some interesting mis-quotations from monks and nuns who didn't quite remember the source, or remembered the words but missed where it came from.
As a result, medieval writers were deeply invested in getting their memories into top form in the areas they needed them to be. They turned first of all to classical authors on rhetoric and oration, the art of arguing well.
In Quintilian, for example, they read of the importance of place: picturing the words on the page in your mind as you read them, and then recalling that visual image as you spoke or thought them later. Ad Herennium, which they thought was Cicero, recommended associating points you wanted to remember with a striking image, and then mentally "placing" them in a mental construction of your house so you would know where to find them again.
And so we have Hugh of St. Victor in the 12th century, instructing his readers to "paint" Noah's Ark in their minds in a very specific manner. A golden cross lies at the ark's center with an X on its right arm and an S on the left arm. X (chi) is the first letter in Christ's name and also the Roman numeral for 10 as in the Commandments; S is the last letter in Christ's name and represents 100 as in the perfection of grace. This image, Hugh says, will fix in the reader's mind that Christ is the beginning and the end, the bridge between the old law and the new.
You know how easy it is to memorize song lyrics? Medieval preachers understood! Fifteenth century French preacher Michel Menot used the notes of the musical scale to preach the horrors of hell:
The damned, he proclaimed, sang these notes in eternity. Sing it back to me, he exhorted his listeners. Remember these notes.
How about lists? I can name the U.S. presidents easily--but only in order, because that's how I learned them. I struggle to do it backwards; alphabetically is impossible. The late Middle Ages loved its lists. A 1475 handbook on how to confess your sins properly outlined its mission thus:
From the nine estranging sins
To the eight ways of holiness
From the seven deadly sins
To the seven sacraments
To the seven gifts of the Holy Spirit
From the six sins against the Holy Spirit
To the six works of mercy, corporal and spiritual
From the five inner sins
From the five outer signs
From the four stirring sins
To the four angelic virtues
To the three godly virtues:
Through these, confession will go well.
Our medieval theorists on memory bridged the literate plane of the educated elite and their surrounding world of people who might recognize the letters in their own name, maybe. This position made them not just need strong memories in the areas they needed strong memories; it made them incredibly conscious of that need. Their sacred mission, in the end, was not to write books of obscure theology: it was to save the souls of medieval Europe. For that, they had to get people to learn and remember the tenets of Christian belief. And for that, they needed to know how to get people to learn and remember them.
We have amazing memories today, but in a lot of ways they aren't focused. I gave examples of things I've memorized quite by accident: through music, through lists, through experience. Medieval writers, with a classical heritage (that they'd memorized...) knew these explicitly, and used them purposefully.
So. Inherently better memories? Not precisely. More trained and purposefully-focused memories? I think Hugh and Michel would agree.
~~
ETA Sources: Mary Carruthers, The Book of Memory and The Craft of Thought are foundational. Kimberly Rivers, Preaching the Memory of Virtue and Vice shows how the theoretical and monastic practice was applied to teaching lay people by late medieval preachers. The Bible mentions pistachios in Genesis 43:11, but you probably already Googled that.
Julius Caesar has an interesting quote on this in the Gallic War (Book VI):
Report says that in the schools of the Druids they learn by heart a great number of verses, and therefore some persons remain twenty years under training. And they do not think it proper to commit these utterances to writing, although in almost all other matters, and in their public and private accounts, they make use of Greek letters. I believe that they have adopted the practice for two reasons — that they do not wish the rule to become common property, nor those who learn the rule to rely on writing and so neglect the cultivation of the memory; and, in fact, it does usually happen that the assistance of writing tends to relax the diligence of the student and the action of the memory.