To date, little evidence has been found associating X with Y.
Up to now, the research has tended to focus on X rather than on Y.
So far, three factors have been identified as being potentially important: X, Y, and Z.
Since 1965, these four economies have doubled their share of world production and trade.

Until recently, there has been little interest in X.
Recently, these questions have been addressed by researchers in many fields.
In recent years, researchers have investigated a variety of approaches to X but …
More recently, literature has emerged that offers contradictory findings about …

The past decade has seen the rapid development of X in many …
Over the past 30 years there has been a significant increase in …
Over the past century there has been a dramatic increase in …
Over the past few decades, the world has seen the stunning transformation of X, Y and Z.